HARTFORD, Conn. (CBS) -- A report on the Newtown school shooting says that the gunman had an obsession with mass murders but that investigators did not discover any evidence he had indicated to others an intention to carry out such a crime.
The summary of the investigation by Connecticut State's Attorney Stephen Sedensky says Adam Lanza was obsessed in particular with the April 1999 shootings at Columbine High School in Colorado.
The report on the Dec. 14 massacre of 20 children and six adults says the question of Lanza's motive may never be answered conclusively.
Lanza killed his mother inside their home before driving to his former elementary school, where he fired off 154 shots with a Bushmaster .223-caliber rifle within five minutes. He killed himself with a handgun as police arrived.
According to the report, authorities seized numerous items from Lanza's Newtown home, including: a spreadsheet listing mass murders by name and information about the incident; computer bookmarks pertaining to firearms, military, politics, mass murder, video games, music, books and ammunition; two videos showing suicide by gunshot; and a five-second dramatized video depicting children being shot.
Sedensky says there is also no clear indication why he chose Sandy Hook as the target for his rampage other than the fact that it was close to his home.
He said Lanza had significant mental health issues that affected his ability to live a normal life and interact with others but did not affect his mental state for the crimes.
"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 says.
Sedensky said police went into the school within minutes of the first shots being fired and that along with the actions of teachers inside saved many children's lives.
“The purpose of the investigation was to determine what crimes had been committed and whether anyone will be prosecuted as a result of those crimes," Sedensky said. "Based on a painstaking investigation it is determined that there will be no arrests or prosecutions. The Connecticut State Police are to be commended for their tireless work on this investigation and their consideration of the families and victims involved."
The report does not include the full evidence file of Connecticut State Police, which is believed to total thousands of pages.
Since the shooting grabbed national headlines, no major gun control legislation has passed Congress, but some states imposed tougher gun laws.
In the past year, many school districts nationwide have beefed up security, but federal efforts to establish background checks for firearm purchases that could limit access for the mentally ill have gone nowhere.
"I think that school security has to be improved. That will be a lesson of the report, and of course longer term mental health initiatives," Conn. Sen. Richard Blumenthal told CBS News correspondent Don Dahler. "What could have been done to reach Adam Lanza before he committed this horrific act?"
In an interview with CBS Evening News anchor Scott Pelley, Nicole Hockley, the mother of victim Dylan Hockley, predicted that Lanza's motive would never be known.
"I would love to know why-- but I think that's a question that is never going to be answered - and I don't expect that to be in the police report - we'll never know what went on in that shooter's mind," she said.