HARTFORD, Conn. — Why Adam Lanza went on his murderous shooting rampage at a Newtown elementary school is still a mystery and may never be known with certainty, prosecutors said Monday in a report that closed out their yearlong investigation.

Lanza, 20, was obsessed with mass murders and the 1999 Columbine High School shooting in particular, but investigators did not find evidence he ever told others of his intentions to carry out such an attack, according to the summary report by the lead investigator, State’s Attorney Stephen Sedensky III.

Lanza killed 20 first-graders and six educators with a semi-automatic rifle inside Sandy Hook Elementary School on Dec. 14.

He also shot and killed his mother inside their home before driving to the school, and took his own life with a handgun as police arrived.

“The obvious question that remains is: `Why did the shooter murder twenty-seven people, including twenty children?’ Unfortunately, that question may never be answered conclusively,” the report said.

Sedensky also said there was no clear indication why Lanza chose Sandy Hook Elementary as the target for his rampage other than the fact that it was close to his home.

The report said Lanza had “significant mental health issues” — in 2005, he was diagnosed with Asperger’s disorder — but “what contribution this made to the shootings, if any, is unknown.”

Asperger’s is an autism-like disorder that is not associated with violence.

Lanza “was undoubtedly afflicted with mental health problems; yet despite a fascination with mass shootings and firearms, he displayed no aggressive or threatening tendencies,” Sedensky wrote.