US investigators said Monday they failed to find a motive for last year's Newtown school massacre, but said the killer had "significant" mental troubles and was obsessed with mass murders.
The official report into the December 14 killings at Sandy Hook Elementary School -- one of America's deadliest mass shootings -- closes the investigation by shedding new light on the extent of Adam Lanza's disturbing behavioral problems.
The 20-year-old was obsessed by firearms, lived as a recluse, suffered from Asperger's Syndrome but refused medication and had his bedroom windows taped over with black trash bags.
The report from Connecticut state attorney Stephen Sedensky said Lanza acted alone but could not conclude why he murdered 20 children and six adults 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," it said.
Lanza first killed his mother, shooting her multiple times in the head as she lay in bed, and then shot his way into the school he once attended in the sleepy town of Newtown.
He killed the principal and school psychologist with a Bushmaster rifle in the hallway and then injured two other members of staff.
He then entered first-grade classrooms eight and 10, killing two adults in each room, 15 children in classroom 8 and five in classroom 10 with the same rifle.
Police arrived less than four minutes after receiving an emergency call. But Lanza killed himself a minute later with a single shot from a Glock 20, 10 mm pistol in classroom 10.
"In fewer than 11 minutes 20 first-grade pupils and six adults had lost their lives," said the report from the state attorney.
More than 300 live rounds were found, along with a semi-automatic pistol on the shooter's body, a semi-automatic shotgun in his car and a rifle on the floor of his mother's bedroom.
All the weapons had been bought legally by Lanza's mother. A Christmas check for another rifle from his mother was also found during a search of their nearby home.
The shooter's GPS device showed that he had driven to different schools in Newtown in the time leading up to the killings, including to Sandy Hook on the afternoon before the murders.
The report said Lanza had "significant mental health issues" and was obsessed with mass murders, especially the 1999 shootings at Columbine High School in Colorado that killed 13 people.
A large number of firearms were found in his home, all legally bought by his mother, 52-year-old Nancy, together with a collection of knives, swords and spears.
Investigators found a spreadsheet detailing mass murders over the years, two videos showing suicide by gunshot, movies depicting mass shootings and bookmarked links about firearms.
Pictures were also found of Lanza holding guns to his head as well, alongside images of hamsters and Lego creations.
There were a number of video games in the house, including US war blockbuster "Call of Duty", three photographs of what appear to be a dead body covered in blood and wrapped in plastic, and newspaper articles about the shooting of school children.
A month before the murders his mother voiced concerns that he hadn't gone anywhere in three months and would communicate with her only by email though they were living in the same house.
Neither his mother nor anyone else was allowed into his room. He was particular about what he ate, disliked birthdays, Christmas and holidays -- not allowing his mother to put up a Christmas tree.
Workmen were told not to ring the doorbell and give notice when using machinery as Lanza had issues with loud noises. He also loathed being touched and was unable to make eye contact.
The Sandy Hook killings renewed emotional debate about gun control in America but resulted in little subsequent reform.