Adam Lanza used a Bushmaster .223 semi-automatic rifle at close range to kill children and adults at Sandy Hook Elementary School in Newtown, Connecticut on Friday.
"I believe everybody was hit more than once," said Dr. H. Wayne Carver, the state of Connecticut's Chief Medical Examiner.
He said the bullets were uniquely damaging and that Lanza's victims died almost immediately.
"The bullets are designed in such a fashion the energy is deposited in the tissue so the bullet stays in," Carver said. He described the wounds as a "very devastating set of injuries."
Two handguns were also found at the scene, but Carver described the Bushmaster as the killer's primary weapon. A fourth weapon was found nearby. The weapons discovered at the school apparently belonged to a family member, possibly his mother, according to authorities.
Lanza, 20, forced his way into Sandy Hook on Friday morning and killed 20 children and six adults before committing suicide. He drove to the school after shooting his mother in the face at their home.
The weapons that police recovered from the scene included a Glock 9-mm handgun, a Sig Sauer 9-mm handgun and a Bushmaster rifle. Police also found .223 shell casings. Lanza was wearing a bullet-proof vest.
The shooter's mother, 52-year-old Nancy Lanza, had five weapons registered to her, including a Glock, a Sig Sauer, and a Bushmaster rifle.
Police said the Glock, the Sig Sauer and the Bushmaster at the school appeared to be registered to a family member. Authorities are currently completing their checks to see which weapons were used in the slayings, to whom they were registered and how they were obtained.
Federal agents are canvassing area gun ranges and gun stories to see if Adam Lanza had visited them for any reason.
The Glock and Sig Sauer are popular with law enforcement officers and the military, and are quickly and easily reloaded.