Adam Lanza forced his way into Sandy Hook Elementary School carrying a Glock, a SIG Sauer, and a Bushmaster rifle, all legally registered to his mother, law enforcement officials said, and all capable of massive killing power.

He chose the rifle over the two handguns, inflicting "devastating" multiple wounds on his 26 victims, said Connecticut's chief medical examiner.

Police said that they found "dozens and dozens" of shell casings from .223 high-velocity rounds inside the school, the type of spent casings that come from bullets used in the Bushmaster rifle.

The lightweight .223 bullets travel at a velocity of about 3,000 feet per second, and after they enter their target, they explode throughout the tissue. As the medical examiner, H. Wayne Carver II, put it at a news conference Saturday, the bullets' "energy stays in the body."

A similar high-powered, semiautomatic Bushmaster rifle was the weapon used by the Washington snipers John Allen Muhammad and Lee Boyd Malvo, who terrorized the Washington metropolitan area in 2002, killing 10 people and critically wounding three.

In 2004, Bushmaster Firearms agreed to contribute $500,000 to a $2.5 million settlement along with codefendant Bull's Eye Shooter Supply, paid to some victims and families of victims of the 2002 Beltway snipers. The company cited mounting legal fees as the reason for settling.

The Bushmaster is one brand of the best-selling type of rifle in America, according to the National Rifle Association, which has been fighting any attempt to reimpose a ban on the AR-15, a civilian modification of the assault rifle used by police and military forces. One can be purchased for about $1,000.

The semiautomatic Glock 9mm handgun, made by an Austrian-based company, is extremely popular with law enforcement officers across the country. A lightweight firearm, the Glock is made of synthetic polymers to ease the recoil and can be quickly and easily reloaded.

It is carried by agents with the FBI and the Drug Enforcement Administration. Two-thirds of U.S. police departments carry the Glock, including officers with the D.C. Police and the New York Police Department.

It is also increasingly the weapon of choice in mass shootings.

The Glock was used in the 2011 shooting at a shopping center in the Tucson, Ariz., area that killed six people and wounded 13, including then-Rep. Gabrielle Giffords. In 2007, a Virginia Tech student used a Glock (along with a .22-caliber semiautomatic pistol) to kill 32 people and wound 17 before killing himself. A .40-caliber Glock was carried by a gunman into a Colorado movie theater in July, where 12 people were killed and dozens more wounded.

"The Glock shows up at mass shootings because, as deranged as these killers are, they clearly prepare thoughtfully to accomplish what they seek to accomplish, killing a lot of people in a short period of time," said Paul Barrett, author of Glock: The Rise of America's Gun.

Barrett said that the Glock was popular because of its large ammunition capacity, its soft trigger pull that allows shooters to fire quickly, and its "dark glamour."

"Within a couple of years after the Glock first appeared in this country in the late 1980s, movie stars were waving it around in Hollywood movies and the most famous hip-hop stars were rapping about the Glock by name, including Tupac Shakur, who was later killed by a Glock," Barrett said.

The 9mm SIG Sauer pistol is also popular with law enforcement and the military because it is lightweight, easy to carry and conceal, and can be easily reloaded. According to SIG Sauer, one-third of U.S. police forces use SIG firearms.

Available for about $300, the handgun fires up to five bullets a second at a velocity of 1,200 feet per second. The SIG Sauer was used in a 1989 shooting in Louisville, Ky., that left eight people dead and 12 wounded.