Adam Lanza killed 26 people inside Sandy Hook Elementary School and took his own life within five minutes of shooting his way into the building Dec. 14, according to a statement by State's Attorney Stephen J. Sedensky III, who is overseeing the investigation. Lanza was found dead in the school wearing military-style clothing.
The evidence seized from Lanza's home and the car he drove to carry out the massacre provided glimpses into the world of the gunman. Until now, prosecutors have made only few details available.
Sedensky said Lanza killed all of his victims inside the school with a Bushmaster .223-caliber model XM15 rifle before taking his own life with a Glock 10 mm handgun. Sedensky said Lanza also had a 9mm Sig Sauer P226 handgun with him inside the school as well as three, 30-round magazines for the Bushmaster and additional ammo for the handguns.
Fourteen rounds were in the magazine when the Bushmaster was recovered by police. There was one round in the chamber.
When the carnage was over, police counted 154 spent bullet casings. A loaded 12-gauge shotgun was also found in the passenger compartment of the Honda Civic Lanza drove to the school.
At the house, investigators found books about autism and Asperger's syndrome as well as an NRA guide to pistol shooting.
In Lanza's bedroom, investigators said they found a smashed computer hard drive and a gaming console. An unnamed source told investigators that Lanza was an avid gamer who played "Call of Duty" and other games and rarely left his home.
Investigators found articles on other shootings and a holiday card containing a check made out to Adam Lanza for the purchase of a firearm, authored by his mother, Nancy Lanza.
According to documents, authorities also found a gun safe with shotgun shells and boxes of bullets. In a bedroom closet, they found ear plugs, a handwritten note regarding ammunition and magazines, paperwork on guns and a metal bayonet.
Police recovered the NRA certificates of both Nancy and Adam Lanza. Adam Lanza's was found in a duffel bag that also contained 50 rounds for a .22 caliber rifle and other ammo, ear and eye protection, binoculars and numerous paper targets.
Authorities also found nine knives and three swords of varying length. They found a military-style uniform in Lanza's bedroom and handwritten notes containing the addresses of local gun shops.
The guns found at the home included a .323-caliber Enfield Albian bolt-action rifle, a .22-caliber Savage Mark II rifle, a BB gun and a .22-caliber Volcanic starter pistol.
Authorities have said it will take until June or later for the investigation to be completed.
ABC News and The Associated Press contributed to this report.