- NEW: Lanza had "had no prior contact with anyone" at Sandy Hook the day of the killings
- The motive behind the Sandy Hook killings remains unknown, report states
- The December 14 shooting left 26 dead, including 20 children
- Gunman Adam Lanza also killed his mother before the school massacre
(CNN) -- Connecticut authorities closed the book on last December's massacre at Sandy Hook Elementary School on Monday without answering what drove the 20-year-old behind the rampage.
A 44-page summary of the second-deadliest shooting in U.S. history outlines how gunman Adam Lanza killed his mother, then stormed the elementary school and killed himself as police arrived. But investigators haven't determined a motive for the assault, which left 20 first-graders and six adults at the grade school dead in less than 11 minutes.
"The evidence clearly shows that the shooter planned his actions, including the taking of his own life, but there is no clear indication why he did so, or why he targeted Sandy Hook Elementary School," Monday's report states.
A U.S. Park Police helicopter removes a man from the Washington Navy Yard on Monday, September 16. At least 12 people were killed in a shooting rampage, according to Washington Police Chief Cathy Lanier. 
Connecticut State Police evacuate children from Sandy Hook Elementary School in Newtown, Connecticut, on December 14, 2012. Adam Lanza opened fire in the school, killing 20 children and six adults before killing himself. Police say he also shot and killed his mother in her Newtown home. 
James Holmes pleaded not guilty by reason of insanity to opening fire July 20, 2012, at the Century Aurora 16 theater in Aurora, Colorado, during the midnight premiere of "The Dark Knight Rises." Twelve people were killed and dozens were wounded. Holmes is charged with 142 counts, including first-degree murder. His trial is scheduled to begin in February 2014. 
A military jury convicted Army Maj. Nidal Hasan on Friday, August 23, of 13 counts of premeditated murder in a shooting rampage at Fort Hood, Texas. During the November 5, 2009 shooting, 13 people died and 32 were injured. 
Jiverly Wong shot and killed 13 people at the American Civic Association in Binghamton, New York, before turning the gun on himself on April 3, 2009, police say. Four other people were injured at the immigration center shooting. Wong had been taking English classes at the center. 
Pallbearers carry a casket of one of Michael McLendon's 10 victims. McLendon shot and killed his mother in her Kingston, Alabama, home, before shooting his aunt, uncle, grandparents and five more people. He shot and killed himself in Samson, Alabama, on March 10, 2009. McClendon left a note saying he put his mother "out of her misery." 
Virginia Tech student Seung-Hui Cho went on a shooting spree on the campus in Blacksburg, Virginia, on April 16, 2007. Cho killed two people at the West Ambler Johnston dormitory and, after chaining the doors closed, killed another 30 at Norris Hall, home to the Engineering Science and Mechanics Department. He wounded 17 people before killing himself. It is the deadliest shooting rampage in U.S. history. 
Mark Barton walked into two Atlanta trading firms on July 29, 1999, and fired shots, leaving nine dead and 13 wounded, police say. Hours later police found Barton at a gas station in Acworth, Georgia, where he pulled a gun and killed himself. The day before Barton had bludgeoned his wife and his two children in their Stockbridge, Georgia, apartment, police say. The children's birth mother and grandmother had been murdered six years earlier in Alabama. Barton was questioned but never charged in that crime. 
Eric Harris, left, and Dylan Klebold entered Columbine High School in Littleton, Colorado, on April 20, 1999, armed with bombs and guns. The students killed 13 and wounded 23 before killing themselves. 
George Hennard crashed his pickup through the plate glass window of Luby's Cafeteria in Killeen, Texas, on October 16, 1991, before fatally shooting 23 people and committing suicide. 
James Huberty shot and killed 21 people, including children, at a McDonald's in San Ysidro, California, on July 18, 1984. A police sharpshooter killed Huberty an hour after the rampage began. 
Prison guard George Banks is led through the Luzerne County courthouse in 1985. Banks killed 13 people, including five of his children, in Wilkes-Barre, Pennsylvania, on September 25, 1982. He was sentenced to death in 1993 and received a stay of execution in 2004. His death sentence was overturned in 2010. 
Officers carry victims across the University of Texas at Austin campus after Charles Joseph Whitman opened fire from the school's tower, killing 16 people and wounding 30. Police officers shot and killed Whitman, who had killed his mother and wife earlier in the day. 
Howard Unruh, a World War II veteran, shot and killed 13 of his neighbors on September 5, 1949, In Camden, New Jersey. Unruh barricaded himself in his house after the shooting. Police overpowered him the next day. He was ruled criminally insane and committed to a state mental institution. 
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Photos: Worst mass shootings in U.S. 
Candles burn next to a lighted tree at a makeshift shrine in Newtown, Connecticut, commemorating the victims of the mass shooting at Sandy Hook Elementary School on December 14, 2012. 
Erica Simmons rings the campus bell at Agnes Scott College in Decatur, Georgia, during a nationwide commemoration December 21, marking a week since the Newtown, Connecticut, mass shooting. Church bells rang out across the country at 9:30 ET Friday to remember those who died in the gun rampage. 
A woman pauses at a streetside memorial during a moment of silence on December 21 in Newtown. 
Connecticut State Police block the road to Sandy Hook Elementary School during a moment of silence on December 21. A week ago, a gunman forced his way into the school and shot and killed 26 people, including 20 children. 
A woman bows her head in Newtown's Sandy Hook village on December 21. 
People observe a moment of silence for the school shooting victims at the Blue Colony Diner in Newtown on December 21. 
Children in Newtown, excluding Sandy Hook Elementary, return to classes on Tuesday, December 18, four days after the shooting at the elementary school. 
U.S. Rep. David Cicilline, D-Rhode Island, holds a news conference with the Brady Campaign to discuss gun violence. In attendance with the Brady Campaign were several survivors of gun violence and family members of victims of gun violence. 
Members of the human rights group Volunteers Against Crime and Corruption light candles showing the names of those killed during the shooting at Sandy Hook Elementary School, during a prayer vigil in front of the U.S. Embassy in Manila, Philippines on December 18. 
Children light candles to pay their respects to the victims of the Newtown, Connecticut, shooting at the main square in Tirana, Albania, on Monday, December 17. The deadly gun rampage at Sandy Hook Elementary School has provoked strong reactions from around the world. 
Protesters march on the National Rifle Association's Capitol Hill lobbyist offices in Washington on December 17. 
New York Mayor Michael Bloomberg speaks out for stronger gun control at a press conference at City Hall on December 17. Bloomberg, co-chair of Mayors Against Illegal Guns, was joined by victims and survivors of gun violence. 
Chris Foye, whose son Chris Owens was killed by a stray bullet in 2009, stands with other survivors and family members of gun violence at Bloomberg's press conference on December 17 in New York. 
People pay their respects on December 17 at a makeshift shrine in Newtown to the victims of Friday's elementary school shooting. Funerals began Monday in the Connecticut town. 
Traders on the floor of the New York Stock Exchange hold a moment of silence on December 17 in honor of the shooting victims. 
Young people mourn at Newtown High School before a memorial service attended by President Obama on Sunday, December 16. 
President Barack Obama waits to speak at an interfaith vigil for the shooting victims from Sandy Hook Elementary School December 16 at Newtown High School. 
Mourners comfort one another December 16 before U.S. President Barack Obama speaks at an interfaith vigil for the shooting victims. 
Two women embrace before the interfaith vigil at Newtown High School on Sunday evening. 
From left: Newtown residents Claire Swanson, Kate Suba, Jaden Albrecht, Simran Chand and New London, Connecticut, residents Rachel Pullen and her son, Landon DeCecco, hold candles at a memorial for victims on Sunday, December 16, in Newtown, Connecticut. 
A young boy walks past Christmas trees set up at a makeshift shrine to the shooting victims in Newtown, Connecticut, on December 16. 
Ty Diaz is kissed by his mother, Yvette, at a memorial down the street from Sandy Hook on December 16. 
Teddy bears, flowers and candles in memory of those killed are left at a memorial down the street from the school on December 16. 
Two teenagers embrace at a makeshift shrine to the victims in Newtown on December 16. 
Nuns pay their respects at a makeshift shrine to the victims on December 16. 
Members of Sisters of Christian Charity go to lay flowers in front of the Sandy Hook Elementary School on December 16 in Newtown. 
A woman receives a hug as she leaves morning service December 16 at Trinity Church in Newtown near the elementary school. 
Parishioners pay their respects to the victims of the elementary school shooting while attending Mass at St. Rose of Lima Roman Catholic Church in Newtown on December 16. 
Police officers honor the victims of the school shooting at the St. Rose of Lima Roman Catholic Church on December 16 in Newtown. 
The Jacksonville Jaguars have a moment of silence in honor of the Sandy Hook Elementary School shooting victims before their game against the Miami Dolphins on December 16. 
A parishioner kneels in front of a makeshift memorial at St. Rose of Lima Roman Catholic Church in Newtown on December 16. 
A police officer removes flowers from a busy intersection on December 16 in Newtown. Police said they were afraid the memorial, left for the victims of the Sandy Hook Elementary School shooting, would cause a traffic hazard. 
A woman hugs her daughter on the steps of Trinity Church on December 16 in Newtown. 
Angel wood cutouts for each of the 27 victims are set up on hillside in Newtown on December 16. 
People in Bangalore, India, hold cards and photographs of the slain at a candlelight vigil outside a Catholic church on December 16. 
J.J. Watt of the Houston Texans shows his glove in remembrance of the victims before the start of a game against the Indianapolis Colts on December 16 in Houston. 
Donna Soto, right, mother of Victoria Soto, the first-grade teacher at Sandy Hook Elementary School who was shot and killed while protecting her students, hugs her daughter Karly while mourning their loss at a candlelight memorial at Stratford High School on Saturday, December 15, in Stratford, Connecticut. 
Firefighters kneel to pay their respects at a makeshift memorial near the school in Newtown on Saturday. 
A child lights a candle at a memorial filled with flowers, stuffed toys and candles outside of Saint Rose of Lima Church near Sandy Hook Elementary School in Newtown, Connecticut on Saturday. 
Candles light up a memorial outside of Saint Rose of Lima Church in Newtown. 
Lucas, Kelly and Michael DaSilva pray and embrace at a makeshift memorial near the school in Newtown. 
People are overcome with emotion Saturday at a makeshift memorial near Sandy Hook Elementary School in Newtown. 
Residents arrive Saturday to pay tribute to the victims of an elementary school shooting in Newtown, Connecticut. 
A couple carry balloons to place at a curbside shrine to in Newtown on Saturday. 
A mother and daughter attend a prayer service at St. John's Episcopal Church in Newtown on Saturday. 
Flowers and signs of sympathy adorn the street leading to Sandy Hook Elementary School. 
Claudia Urbiana and daughter Jocelyne Cardenas, left, hug outside of the entrance to the Sandy Hook school. 
A makeshift memorial with flowers, stuffed toys and candles sit outside Saint Rose of Lima Roman Catholic Church in Newtown, Connecticut, on Saturday. 
A man bows his head as he stands at a makeshift memorial, outside Saint Rose of Lima Roman Catholic Church in Newtown on Saturday, December 15. 
New Jersey resident Steve Wruble, who was moved to drive out to Connecticut to support local residents, grieves for victims at the entrance to Sandy Hook village in Newtown on Saturday. 
People attend a prayer service in Newtown on Saturday to reflect. 
A mother hugs her children after paying tribute to the victims in Newtown on Saturday, December 15. 
Newtown High School student Trevor Lopez stands outside of a church where residents have come to pray and reflect on Saturday 
Andrea Jaeger places flowers and a candle at a makeshift memorial outside a firehouse near Sandy Hook Elementary School on Saturday. 
The U.S. flag flies at half-staff above the White House on Saturday. 
Ken Kowalsky and his daughter Rebecca, 13, embrace while standing at the end of the road leading to Sandy Hook Elementary School on Saturday. 
A woman puts a flower near crosses planted by Rio de Paz (Rio of Peace), in memory of the victims of the Sandy Hook Elementary school shooting on Copacabana beach in Rio de Janeiro on Saturday. 
A woman sits during a service at the Asylum Hill Congregational Church in Hartford, Connecticut on Friday, December 14. 
Fans at the NBA game between the Utah Jazz and the Phoenix Suns participate in a moment of silence for the victims of the Newtown shooting on Friday in Phoenix. 
An overflow crowd listens to a church service held at the St. Rose of Lima Catholic Church in Newtown on December 14. 
People gather for a prayer vigil at St. Rose Church on Friday. 
People gather for a vigil outside the White House in Washington following the Connecticut elementary school shooting on Friday. 
Candles burn as people gather for a vigil outside the White House. 
Hartford, Connecticut, Mayor Padro Segarra speaks emotionally about the students and teachers who died earlier in the day at Sandy Hook Elementary School in nearby Newtown at a candlelight vigil at Bushnell Park in Hartford on Friday. 
Cynthia Alvarez is comforted by her mother, Lilia, as people gather for a prayer vigil at St. Rose of Lima Roman Catholic Church in Newtown. 
People gather for a prayer vigil at St. Rose Church in Newtown. 
People gather for a prayer vigil at St. Rose Church. 
People gather inside the St. Rose Church to remember the shooting victims on Friday 
A woman bows her head during a vigil for the shooting victims at St. Rose Church. 
People gather in the St. Rose Church for a memorial service Friday. 
Women comfort each other during the vigil at St. Rose Church. 
A woman looks on during the vigil at St. Rose Church. 
People hug outside of the Newtown United Methodist Church on Friday, near the site of the shootings at the Sandy Hook school. 
A flag at the U.S. Capitol flies at half-staff after President Barack Obama ordered the action while speaking from the White House. Obama called for "meaningful action" in the wake of the school shooting. 
Julie Henson of San Francisco joins other people outside the White House to participate in a candlelight vigil on Friday. 
Obama wipes tears as he makes a statement in response to the shooting on Friday. 
Washington resident Rachel Perrone, left, and her 5-year-old son, Joe, center, join others outside the White House in a candlelight vigil. 
Faisal Ali, right, of Colorado Springs, Colorado, joins the vigil outside the White House. 
People gather outside the White House to participate in a candlelight vigil. 
Supporters of gun control hold a candlelight vigil for victims of the shooting outside the White House. 
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Reaction to Newtown school killings 
Report: Sandy Hook motive inconclusive Lanza "had significant mental health issues that affected his ability to live a normal life and to interact with others," the report states. But it adds, "What contribution this made to the shootings, if any, is unknown as those mental health professionals who saw him did not see anything that would have predicted his future behavior."
There were no drugs in his system at the time of the killings, according to autopsy results included in the report. Lanza "did not drink alcohol, take drugs, prescription or otherwise, and hated the thought of doing any of those things," investigators found.
"With the issuance of this report, the investigation is closed," Monday's report concludes.
The killings in Newtown, about 80 miles outside New York, happened less than five months after a similar bloodbath at a movie theater in Aurora, Colorado, outside Denver. The killings ignited a nationwide debate over gun violence, school safety and mental health.
Lanza had "an obsession" with mass murder and "a strong interest in firearms," the report states. On his computer, investigators found a spreadsheet detailing mass murders over the years, two videos depicting gunshot suicides, two pictures of Lanza pointing guns at his own head, and movies and a video game depicting school shootings.
But violent video games weren't his only pastime: He also was obsessed with the dancing game "Dance Dance Revolution." He not only played it at home, he went to a local theater that had a version in its lobby nearly every weekend, playing it for as long as 10 hours at a time.
Another hard drive found in his home appeared to have been intentionally damaged, and investigators were unable to recover anything from the device.
And investigators found "a large number" of guns in the home. All of them had been bought by his mother, Nancy Lanza, who grew up with firearms and "thought it was good to learn responsibility for guns," the report states. Both she and Adam Lanza shot pistols at a local range, where Adam "was described as quiet and polite."
Nancy Lanza, 52, "took care of all of the shooter's needs" and "worried about what would happen to the shooter if anything happened to her," according to the report.
"The shooter was particular about the food that he ate and its arrangement on a plate in relation to other foods on the plate," it recounts. "Certain types of dishware could not be used for particular foods. The mother would shop for him and cook to the shooter's specifications, though sometimes he would cook for himself."
Nancy Lanza did her son's laundry every day, but was not allowed into his room -- "No one was allowed in his room," where the windows were covered with black plastic trash bags, the report notes. Adam Lanza "disliked birthdays, Christmas and holidays," forbidding his mother from putting up a Christmas tree: "The mother explained it by saying that shooter had no emotions or feelings."
One person described Lanza's relationship with his mother as "strained," while another told investigators he didn't appear to have "an emotional connection to his mother." But others said Nancy Lanza "was the only person to whom the shooter would talk."
The morning of the massacre, Lanza shot his mother several times in the head with a .22-caliber rifle while she lay in bed, the report states. He then headed to Sandy Hook, where he had attended grade school -- but "as best as can be determined, the shooter had no prior contact with anyone in the school that day."
Four guns and more than 300 rounds of ammunition were found with Lanza, including the .223-caliber Bushmaster semi-automatic rifle used at the school and the 10mm Glock pistol he used to kill himself. Another pistol and a shotgun weren't used, the report states.
Monday's report is separate from a much longer evidence file that Connecticut State Police will release at an unspecified date. The family of Victoria Soto, a teacher who shielded her students before being shot to death, said the release is "yet another blow that our family has been dealt."
A statement from the family said, "While others search for the answer as to why this happened, we search for the how. How can we live without Vicki? How do we celebrate Christmas without Vicki? How do we go on every day missing a piece of our family? Those are the questions we seek the answers for. There is nothing in the report that will answer those for us."
Connecticut Gov. Dannel Malloy said the report's release "will no doubt be difficult" for the relatives of those killed at Sandy Hook.
"But if there is one thing that I believe we must do, it's that we must honor the lives that were lost by taking steps to protect ourselves from another horror like this," Malloy said. "I hope that the information in this summary and in the supporting documents that will be released by the State Police takes us closer to that goal."
Connecticut panel: Release 911 calls in Newtown massacre
Victims' family members were informed of the report, said Mark Dupuis, a spokesman for Danbury State's Attorney Stephen Sedensky, whose office conducted the investigation.
Dupuis declined to provide details about when, where, and how the families were given the details of the report.
"We are sensitive to the needs of the families, and those needs are being addressed," Dupuis said.
Haunted by the Newtown massacre, police officer faces firing over PTSD
After the Newtown killings, a handful of states -- Connecticut among them -- passed new regulations on background check limits, magazine capacity and types of firearms legally available. But efforts to pass even limited legislation at the federal level were thwarted by a Republican filibuster in the Senate; the Obama administration then announced limited executive reforms in place of tougher laws.
CNN's Susan Candiotti, Rande Iaboni and Ronni Berke contributed to this report.
