Source: Colorado shooter James Holmes had 100-round rifle magazine

AURORA, CO - Authorities in the Colorado movie theater massacre found an AR-15 rifle drum magazine Friday capable of carrying 100 rounds, according to a law enforcement source working the investigation.

The discovery indicates the shooter may have been prepared to target a high number of victims when he entered a crowded theater and opened fire.

Police took suspect James Holmes, 24, into custody Friday after they say he set off two devices and sprayed the theater with bullets from an AR-15, a 12-gauge shotgun and at least one of two .40-caliber handguns police have recovered. Investigators are confident that Holmes acted alone, police said.

All the weapons, said the law enforcement source, were purchased legally at gun stores in the past six months.

Purchasing a 100-round magazine for an AR-15 is unusual, weapons experts said. The AR-15 is designed for easy reloading. "Even without the grand-sized mags, many people who are practiced can reload in 1½ to 2 seconds," said Steven Howard, a Michigan attorney and security and firearms expert.

Howard said a crowded theater offers an ideal location for a gunman -- particularly one armed with a shotgun -- to target dozens of victims. A large room such as a theater -- where people are seated in rows -- makes moviegoers easy targets, even for an inexperienced shooter, Howard said.

Screaming, panicked moviegoers scrambled to escape from the gunman, who wore a gas mask and randomly shot as he walked up the theater's steps, witnesses said.

"The very nature of theaters makes them perfect killing zones," said Howard.

A shotgun would do more damage among a tightly packed theater audience because its ammunition comes out of the weapon in a reverse funnel shape and would disperse across a wider area, according to Howard.

In a theater scenario, he said, "so many people's heads are lined up next to each other that if you fire down these rows of people ... one blast is going to kill or seriously injure 10 or 15 people, depending on a number of variables."

An AR-15 nonmilitary model, Howard said, shoots one bullet at a time and that bullet "may go through two people."

No weapons expertise would be necessary to execute an attack such as the one in Aurora, said Howard, a former U.S. Border Patrol agent. "Any idiot can do it."

Holmes was apprehended outside his white Hyundai parked in the back of the theater, police said. Three of the weapons were in the car and one was left at the scene inside the theater, said authorities.

Howard offered these details about the weapons police say were used in the attack:

-- AR15 rifle: A rifle that commonly fires bullets .223 millimeter in diameter. Originally manufactured by Colt before its patent ran out, but now also made by others. Cost: around $900 and up. Its military version is a machine gun, which allows a shooter to fire continuously by holding the trigger down. The nonmilitary AR-15 is a semi-automatic which shoots one bullet at a time, with each squeeze of the trigger. Ammunition magazines for the AR-15 commonly hold five to 15 rounds.

-- 870 Remington 12-gauge shotgun: One of the most popular shotguns in history. It can be modified to have a short or long barrel.

-- .40-caliber Glock handgun: Common weapon for police. Too big to put in pants pockets, but not too big to stick inside a waistband.

The attacker also used tear gas in the attack, according to police. Tear gas and smoke bombs would "add to the panic" and make victims less likely to be able to defend themselves against an attacker, Howard said.

The dozens of wounded victims in the attack could well have been hit by ricocheting bullets. "Once bullets hit a wall, they tend to ricochet down a wall, but they don't bounce like pool balls like a lot of people think and like the movies show. They follow the last surface they hit," Howard said.

Carpeted walls, which are common in movie theaters would reduce the risk of ricochets, said ballistics expert Jon Nordsby. Bullets that have first passed through objects such as theater chairs might cause worse wounds. The bullets start tumbling after they first penetrate the object. A tumbling bullet will tear more flesh when it hits, he said.

Crime scene investigators will likely pay close attention to the location of ammunition shell casings and bullets to learn more details about the attack. They also will likely document the locations where victims fell to determine the trajectory of the bullets.

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