The prosecutor investigating the Sandy Hook elementary school shooting has closed his inquiry with no firm explanation why Adam Lanza planned and executed 27 people, including his own mother, last year.

The final report, which was posted online (pdf) today, is the first attempt to provide a comprehensive breakdown of what happened on Dec. 14, 2012, and the life that Lanza led before that day. It found that Lanza acted alone, telling no one of his plans.

It also uncovered no motive. None of the mental health professionals who evaluated him saw anything to suggest he would become violent, the report found. But the report does show a reclusive young man who already struggled to interact with others and became increasingly isolated in the two years — and in particular the few months — before the attack.

Lanza had been diagnosed with Asperger’s Disorder in 2005, the report noted, and had extreme anxiety and “significant social impairments.” He struggled with obsessive-compulsive tendencies, which led to multiple daily rituals. He washed his hands repetitively, and changed his clothes so often that his mother did his laundry every day. Lanza, who couldn’t bear to be touched, was also particular about his food, and his mother, Nancy, complied with his requirements, buying and cooking meals to his specifications.

Lanza also lacked empathy, the report said. He had insisted his mother get rid of their cat, and he hated holidays, forbidding her to put up a Christmas tree. “The mother explained it by saying that [Lanza] had no emotions or feelings,” the report said.

In interviews, some people told investigators that his mother was the only person Lanza spoke to, but even that relationship eventually withered. “[W]hen his mother asked him if he would feel bad if anything happened to her, he replied, ‘No,’” the report said. Adam told someone that their relationship was strained because Nancy’s behavior was “not rational.” She was never allowed to enter his room.

By November 2012, Nancy was concerned about her son. He hadn’t gone anywhere in three months, and communicated with her only by e-mail, even though they lived under the same roof, the report said.

She never betrayed any fear that he might become violent and kept looking for ways to engage him, according to the report.  She considered buying him another computer, and planned to give him money for a CZ83 pistol, writing out a check to her son with the subject “CZ83,” dated Christmas day.

Inside his room, Lanza played a lot of video games — some violent, some not — and became increasingly obsessed with school shootings, compiling a spreadsheet of these incidents. Officials also found images of Lanza holding various guns to his head, along with pictures of himself with a rifle, shotgun and his pockets stuffed with magazines, and a digital video, which was a five-second dramatization of children being shot.

On Dec. 10, four days before the shooting, Nancy told a friend that Lanza had bumped his head and was bleeding, but she didn’t say why and added that he was all right. She left for a trip to New Hampshire, leaving the house stocked with his favorite foods.

She returned at 10 p.m. on Dec. 13, the night before the shooting. Earlier that day, Lanza had driven his black Honda Civic down to the Sandy Hook area and back, a 23-minute trip, according to a GPS found in the home. It isn’t clear whether he passed by the elementary school.

Sarah Childress, Former Series Senior Editor, FRONTLINE