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, he said.
"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 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.
Since the shooting grabbed national headlines, no major gun control legislation has passed Congress despite a major push by the Obama administration.
Federal efforts to establish background checks for firearm purchases that could limit access for the mentally ill also have made little headway in the face of a powerful gun lobby.
