A final report released by the state's attorney for the Judicial District of Danbury on Monday found that Sandy Hook Elementary School shooter Adam Lanza had no apparent motive for gunning down 27 people on Dec. 14, 2012, in Newtown, Conn., including 20 children, six school officials, and Lanza's mother.

Lanza's struggle with mental illness — documented at length in the final pages of the report — was "no defense to his conduct," according to investigators. Beyond proximity, there was no clear reason why Lanza chose Sandy Hook Elementary School as his target.

"What we do know is that the shooter had significant mental health issues that, while not affecting the criminality of the shooter's mental state for the crimes or his criminal responsibility for them, did affect his ability to live a normal life and to interact with others, even those to whom he should have been close," wrote Danbury State's Attorney Stephen J. Sedensky III.