Lanza left behind journals, which state police turned over to the FBI for analysis, but if investigators have any ideas about his motive, they aren't saying.
"Why is the big unanswered question," said Nicole Hockley, whose son Dylan was among the children killed at Sandy Hook Elementary School. "So I have to come to grips with the fact that I might never know why he did what he did. But knowing how he did it and what we could do to prevent someone else doing that, those are valuable lessons that we can learn."
Warrants released Thursday provide the most insight to date on the world of the 20-year-old gunman, a recluse who played violent video games in the Newtown home where he lived with his mother.
On the morning of Dec. 14, he took four guns but left behind firearms, knives and more than 1,600 rounds of ammunition at his spacious, colonial-style home.
He loaded the weapons into his car, drove to his former elementary school, and within five minutes of blasting his way into the building, he fired off 154 shots with a Bushmaster .223-caliber rifle as he gunned down the first-graders and educators. He killed himself with a final shot from a Glock handgun.
The weapons used in the shooting had all apparently been purchased by Lanza's mother, according to prosecutor Stephen J. Sedensky III, who said in a statement accompanying the warrants that the gun locker was open when police arrived at the house and there was no sign it had been broken into. Lanza shot his mother, Nancy Lanza, in her bed before carrying out the massacre.
Connecticut Gov. Dannel P. Malloy expressed incredulity over the access that the troubled young man had to a cache of weapons.
"There are parts of this story that are unfathomable," he said. "How anyone would have maintained that household that way is difficult to understand."
Sedensky, the prosecutor overseeing the investigation, said Connecticut State Police, Newtown police and other state and federal agencies are still compiling reports and statements from witnesses and testing physical and digital evidence.
"No conclusions have been reached and no final determinations have been made," he wrote.
Investigators found weapons at the Lanza house including a 7-foot pole with a blade on one side and a spear on another, a metal bayonet, three samurai swords, a .323-caliber bolt-action rifle, a .22-caliber Savage Mark II rifle and a .22-caliber Volcanic starter pistol. A gun safe was found in Adam Lanza's bedroom along with a military-style uniform. Literature seized from the house included a news article on a 2008 shooting at Northern Illinois University and an NRA guide to pistol shooting.
On the day of the massacre, Lanza took two loaded handguns to the school along with the Bushmaster rifle that he used to kill all the victims inside the school. A fourth gun, a loaded 12-gauge Saiga shotgun, was found in the passenger compartment of the Honda Civic Lanza drove to the school with 70 shotgun rounds, according to a warrant.
Lanza went through six 30-round magazines for the Bushmaster, although half of them were not completely empty, and police said he had three other 30-round magazines in addition to one that was in the rifle.