Inside the rambling, pale-yellow Colonial-style home in a Connecticut suburb, Adam Lanza lived amid a stockpile of disparate weaponry and macabre keepsakes: several firearms, more than 1,600 rounds of ammunition, 11 knives, a starter pistol, a bayonet, 3 samurai swords. He saved photographs of what appeared to be a corpse smeared in blood and covered in plastic, as well as a newspaper clipping that chronicled a vicious shooting at Northern Illinois University.

A gun safe was in what investigators believed was his bedroom. Among his clothing was a military-style uniform. There was also a holiday card that contained a check made out to Mr. Lanza, 20, and signed by his mother. Investigators suggested that the money had been intended to buy a gun.

The disturbing details of Mr. Lanza’s possessions were disclosed on Thursday for the first time since he carried out the massacre at Sandy Hook Elementary School in Newtown, Conn., one of the deadliest school shootings in the nation’s history. The information was included in search warrants and related affidavits connected to the investigation into the Dec. 14 attack, when he killed 20 first graders, 6 educators, his mother and himself.

The inventory of the house, combined with interviews conducted over several weeks with law-enforcement officials and people who crossed paths with the Lanza family, afford a somewhat fuller picture of the dark corners of Mr. Lanza’s mind.