The lone gunman who killed 26 people during a massacre last year in Newtown, Connecticut, was obsessed with guns, school shootings and harming children, investigators said.

But the State Attorney's office said in a final report on the Dec. 14, 2012, rampage that their investigation failed to answer the most significant question they had.

“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,” the report says.

Investigators said 20-year-old Adam Lanza committed murder under special circumstances 26 times and attempted murder under special circumstances twice during the Dec. 14, 2012, rampage, which authorities said lasted less than 11 minutes.

He had shot his mother to death in her own bed before driving himself to the school, which he had attended until fifth grade, in 2003.

The other felonies he committed during the execution of his crime and which triggered the special circumstances were assault, burglary, risk of injury to a minor, possession of a weapon on school grounds and carrying a pistol without a permit.

“The shooter intentionally armed himself heavily, drove to SHES, parked in a manner out of direct sight of the front door, shot his way into the building and immediately killed those who confronted him as well as those in classrooms 8 and10,” authorities said. “The evidence found at his home on the digital media further support his intentions to kill, both at the school and with his mother. Further the manner in which he killed his mother reflects the shooter’s intent to kill her."

Investigators said the first police officers arrived at the school within four minutes of a 911 call that was placed after the shooter entered the school and shot the principal and school psychologist as they out of an office to investigate the noise.

He then entered two first grade classrooms and opened fire with a Bushmaster X15-E2S rifle, killing 15 children and two adults in one classroom and five children and two adults in the second.

The shooter then used a Glock 20 10-mm pistol to fatally shoot himself in the head, the report shows.

“In this case the shooter’s mental status is no defense to his conduct as the evidence shows he knew his conduct to be against the law,” the report says. “He had the ability to control his behavior to obtain the results he wanted, including his own death.”

Investigators said the shooter removed the GPS from his car, used ear plugs, destroyed his computer’s hard drive and waited for his mother to return the night before from a trip to New Hampshire.

He shot his mother, Nancy Lanza, with a .22-caliber Savage Mark II rifle that police later found at their home, police said.

“The existence of an extreme emotional disturbance for which there is a reasonable explanation or excuse is also not present in this case,” the report says. “It is clear that the shooter planned his crimes in advance and was under no extreme emotional disturbance for which there was a reasonable explanation or excuse.”

Officers also recovered the Bushmaster rifle found next to the shooter’s body, and they said all the 5.56-mm shell casings found at the school had been fired from that weapon.

The Glock was found near his body and a Sig Sauer P226 9-mm body was found on his person, investigators said.

Police also found an Izhmash Saiga-12 12-gauge semi-automatic shotgun in the trunk of the shooter’s car parked outside in the lot.

There was no evidence that weapon had been fired, and police said they found some live shotgun shells inside the school that were determined to belong to other police officers who responded to the shooting.

The report does not list the names of the children who were killed in the massacre, recite the 911 calls or describe witness statements of what happened inside the school, which police said was consistent with exceptions to the Freedom of Information Act.

Police said they heard the final gunshot that felled the shooter as they arrived at 9:40 a.m.

In the moments police arrived, they stopped a man who turned out to be a parent running alongside the school with a cell phone, two reporters in some nearby woods and a man who was working nearby and was alerted by his cell phone of the emergency situation at the school.

Investigators determined the shooter acted alone, and indeed had few close friends and remained emotionally distant from his family.

Authorities said the shooter had not been in contact with his father or an older brother since about 2010, and he communicated primarily by email with his mother in the home they shared.

They said the shooter kept detailed spreadsheet records of other school shootings and seemed particularly fascinated by the 1999 Columbine High School massacre.

He taped black trash bags over his second-floor bedroom and computer room windows, apparently watched videos or movies depicting mass shootings, human suicides and the shooting deaths of children and took photos of himself holding a handgun and rifles to his head.

The shooter also played a number of video games, including first-person shooter games, and engaged in lengthy Dance Dance Revolution marathons.

Police said they found screen shots and videos of him playing the game, which requires players to dance along with provided music and cues.

They said he frequently played at a local movie theater for up to 10 hours a day Friday through Sunday.

The shooter was diagnosed with Asperger syndrome at 2005, which police said may have explained his detached demeanor.

“The shooter disliked birthdays, Christmas and holidays,” investigators said. “He would not allow his mother to put up a Christmas tree. The mother explained it by saying that shooter had no emotions or feelings. The mother also got rid of a cat because the shooter did not want it in the house.”

Investigators said they found evidence the shooter's mother intended to buy him a CZ 83 pistol last year for Christmas.

The shooter refused to take any suggested medication and disdained all types of drugs or alcohol, investigators said.

“(His mother) also described the shooter as being unable to make eye contact, sensitive to light and couldn’t stand to be touched. Over time he had multiple daily rituals, an inability to touch door knobs, repeated hand washing and obsessive clothes changing, to the point that his mother was frequently doing laundry,” the report says.

But authorities said they found no evidence from anything they seized or in any witness statements that would explain why he chose to kill his mother, heavily arm himself and drive to the nearby school to carry out his horrific plan.

[Image via Agence France-Presse]