The Connecticut State's Attorney has released their report on the Sandy Hook shooting [pdf]. The case has been declared closed at this point.
There aren't any substantial revelations about Lanza's motives. He was not found to be taking medication, nor was he under the influence of drugs at the time. He does not appear to have told anyone his intentions in advance.
Regarding the search of his home, other weapons were found. All of them were registered to his mother in accordance with state and federal laws. A computer was found in Lanza's bedroom, but he appears to have damaged it to make the contents unreadable.
(…) investigators found a computer hard drive that appeared to have been intentionally damaged. To date, because of the extensive damage, forensic experts have not yet been able to recover any information from that hard drive.
In a typical criminal case, the investigation would remain open when potentially important evidence was still being examined. Given the improbability of any information being recovered from the damaged hard drive, this outstanding piece of evidence is not preventing the closure of this case now.
He had a GPS with routes to and from other local schools in the area. The report also lists video games found in the house, which included titles like Call of Duty and Doom. He was also apparently a fan of Dance Dance Revolution, so the argument that video games were contributors to his behavior becomes more than a bit strained.
He did reading on the Amish school shooting and the Northern Illinois University incident, and "photocopied newspaper articles from 1891 pertaining to the shooting of school children" were found among his effects. He seems to have researched the Columbine shooting extensively, and there were photos on a hard drive (presumably a different one) in which Lanza held handguns and rifles to his own head. This is similar to some of Seung-Hui Cho's behavior and interests.
Unlike Cho, however, he left behind no writings to indicate motive. He appears to exhibited many of the classic facets of autism and was unemotional towards others, but his history shows no prior inclination to violence.
At the end of it, we have a collection of facts, but we're still no closer to knowing why he did this. Despite the morbid politics and finger-pointing, we've yet to see the national conversation we were promised about mental illness. Until that happens, we will have no way of preventing such tragedies in the future.