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(CBS News) The prosecutor who led the investigation into the Newtown school shooting says it did not determine a motive for the attack.

State’s Attorney Stephen Sedensky III says there is also no clear indication why he chose Sandy Hook Elementary School as the target for his rampage other than the fact that it was close to his home.

He said the gunman, 20-year-old Adam 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 reports says: “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, despite the collection of extensive background information on the shooter through a multitude of  interviews and other sources. 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 comes nearly one year after the massacre of 20 children and six women inside Sandy Hook Elementary School.

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,”.

On Dec. 14, 2012, 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.

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.