Story highlights "It was a pretty rapid pace of fire in that theater," the police chief says

More than 6,000 rounds of ammunition was bought through online purchases, he adds

AR-15, Glock handgun and shotgun used in deadly movie attack

Expert: Shotgun blast across row of theater seats could kill or wound 15 people

Authorities in the Colorado movie theater massacre found an AR-15 rifle drum magazine Friday capable of carrying 100 rounds, and the police chief said thousands of rounds of ammunition for various weapons had been bought online in the weeks prior to the shooting.

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.

Aurora, Colorado, Police Chief Dan Oates said Friday night that Holmes had purchased four guns at "local metro gun shops," and bought over 6,000 rounds of ammunition through online transactions.

Assessing what authorities have pieced together about the shooting and the weapons recovered, Oates told reporters, "As far as we know, it was a pretty rapid pace of fire in that theater."

Chad Weinman, CEO of TacticalGear.com of Chesterfield, Missouri, told CNN earlier that his company had a receipt matching Holmes' name and his Aurora address.

Photos: Colorado movie theater massacre Photos: Colorado movie theater massacre The public gets its first glimpse of James Holmes, then 24, the suspect in the Colorado theater shooting during his initial court appearance July 23, 2012. With his hair dyed reddish-orange, Holmes, here with public defender Tamara Brady, showed little emotion. He is accused of opening fire in a movie theater July 20, 2012, in Aurora, Colorado, killing 12 people and wounding 70. Holmes faces 166 counts, almost all alleging murder or attempted murder. He has pleaded not guilty by reason of insanity. More photos: Mourning the victims of the Colorado theater massacre Hide Caption 1 of 45 Photos: Colorado movie theater massacre Police release the official photo from Holmes' booking after the shooting. Hide Caption 2 of 45 Photos: Colorado movie theater massacre Holmes often had a blank stare during his July 23, 2012, court appearance, seeming to be in a daze. Hide Caption 3 of 45 Photos: Colorado movie theater massacre Victims and their relatives and journalists watch the proceedings in 2012. Hide Caption 4 of 45 Photos: Colorado movie theater massacre Flags fly at half-staff on July 23, 2012, at the Arapahoe County Courthouse in Centennial, Colorado, where the movie theater shooting suspect had his first court appearance. The murder counts against Holmes carry a possible death penalty. Hide Caption 5 of 45 Photos: Colorado movie theater massacre Arapahoe County District Attorney Carol Chambers talks to reporters July 23, 2012, before heading into the courthouse. The murder counts against Holmes carry a possible death penalty. Hide Caption 6 of 45 Photos: Colorado movie theater massacre Family members of the victims arrive at the courthouse July 23, 2012, for the suspect's first court appearance. Hide Caption 7 of 45 Photos: Colorado movie theater massacre The Century Aurora 16 multiplex in Aurora becomes a place of horror after a gunman opened fire July 20, 2012, in a crowded theater. Hide Caption 8 of 45 Photos: Colorado movie theater massacre Holmes is accused of opening fire during a midnight screening of the Batman movie "The Dark Knight Rises." Holmes purchased four weapons and more than 6,000 rounds of ammunition, police say. Hide Caption 9 of 45 Photos: Colorado movie theater massacre Police investigate outside the Century 16 multiplex July 21, 2012, a day after the mass shooting. Hide Caption 10 of 45 Photos: Colorado movie theater massacre Agents search the suspect's car outside the theater. Hide Caption 11 of 45 Photos: Colorado movie theater massacre Aurora police escort a sand-filled dump truck containing improvised explosive devices removed from Holmes' booby-trapped apartment on July 21, 2012. Authorities have said they believe the suspect rigged his place before leaving for the movie theater. Hide Caption 12 of 45 Photos: Colorado movie theater massacre Police break a window at the suspect's apartment July 20, 2012, in Aurora. Hide Caption 13 of 45 Photos: Colorado movie theater massacre Law enforcement officers speak with Colorado Gov. John Hickenlooper, center, outside the suspect's apartment July 21, 2012. Hide Caption 14 of 45 Photos: Colorado movie theater massacre Law enforcement officers prepare to disarm the booby-trapped apartment July 21, 2012. Hide Caption 15 of 45 Photos: Colorado movie theater massacre Officials tow cars outside Holmes' apartment July 21, 2012. Police disassembled devices and trip wires set up in the apartment. Hide Caption 16 of 45 Photos: Colorado movie theater massacre Officers prepare to place an explosive device inside the apartment. Hide Caption 17 of 45 Photos: Colorado movie theater massacre Debris flies out a window, right, after law enforcement officers detonate an explosive device inside the apartment July 21, 2012. Hide Caption 18 of 45 Photos: Colorado movie theater massacre People mourn the victims during a vigil behind the theater where a gunman opened fire on moviegoers in Aurora. Hide Caption 19 of 45 Photos: Colorado movie theater massacre A woman grieves during a vigil for victims behind the theater. Hide Caption 20 of 45 Photos: Colorado movie theater massacre A distraught woman receives counseling from the Rev. Quincy Shannon, left, in front of Gateway High School in Aurora, where the families of the missing met after the shooting. Hide Caption 21 of 45 Photos: Colorado movie theater massacre Lin Gan of Aurora holds back tears as she speaks to reporters about her experience in the Century 16 theater. Hide Caption 22 of 45 Photos: Colorado movie theater massacre People embrace before a vigil for victims behind the theater where a gunman opened fire on moviegoers. Hide Caption 23 of 45 Photos: Colorado movie theater massacre Investigators work on evidence near the apartment of James Holmes on July 20, 2012. Hide Caption 24 of 45 Photos: Colorado movie theater massacre Members of the Aurora Police Department SWAT unit walk near the Holmes' apartment. Hide Caption 25 of 45 Photos: Colorado movie theater massacre Television news crews gather in front of the home of Robert and Arlene Holmes, parents of suspect James Holmes, in San Diego on July 21, 2012. Hide Caption 26 of 45 Photos: Colorado movie theater massacre A popcorn box lies on the ground outside the Century 16 movie theater. Hide Caption 27 of 45 Photos: Colorado movie theater massacre Adariah Legarreta, 4, is comforted by her grandmother Rita Abeyta near the Century 16 Theater in Aurora. Hide Caption 28 of 45 Photos: Colorado movie theater massacre Jessica Ghawi, an aspiring sportscaster, was one of the victims. Hide Caption 29 of 45 Photos: Colorado movie theater massacre A woman waits for news outside Gateway High School, a few blocks from the scene of the shooting at the Century Aurora 16. Hide Caption 30 of 45 Photos: Colorado movie theater massacre Aurora police chief Daniel J. Oates speaks at a press conference near the Century 16 Theater on July 20, 2012. Hide Caption 31 of 45 Photos: Colorado movie theater massacre Agents search the trash container outside the suspect's apartment in Aurora. Hide Caption 32 of 45 Photos: Colorado movie theater massacre A Federal ATF officer carries protective gear onsite at the home of the shooting suspect. Hide Caption 33 of 45 Photos: Colorado movie theater massacre President Obama speaks on the shootings at a July 20, 2012, event in Fort Myers, Florida. Hide Caption 34 of 45 Photos: Colorado movie theater massacre Moviegoers are interviewed at the Century Aurora 16. Hide Caption 35 of 45 Photos: Colorado movie theater massacre Officers gather at the theater July 20, 2012. Hide Caption 36 of 45 Photos: Colorado movie theater massacre Investigators were a common sight at the theater on July 20, 2012. Hide Caption 37 of 45 Photos: Colorado movie theater massacre Authorities gather at the shooting suspect's apartment building in Aurora. Police broke a second-floor window to look for explosives the suspect claimed were in the apartment. Hide Caption 38 of 45 Photos: Colorado movie theater massacre Screaming, panicked moviegoers scrambled to escape from the black-clad gunman, who wore a gas mask and randomly shot as he walked up the theater's steps, witnesses said. Hide Caption 39 of 45 Photos: Colorado movie theater massacre Onlookers gather outside the Century Aurora 16 theater. Hide Caption 40 of 45 Photos: Colorado movie theater massacre A woman sits on top of her car near the crime scene. Hide Caption 41 of 45 Photos: Colorado movie theater massacre Police block access to the Town Center mall after the shooting. Hide Caption 42 of 45 Photos: Colorado movie theater massacre Cell phone video taken by someone at the theater showed scores of people screaming and fleeing the building. Some, like this man, had blood on their clothes. Hide Caption 43 of 45 Photos: Colorado movie theater massacre Witnesses told KUSA the gunman kicked in an emergency exit door and threw a smoke bomb into the darkened theater before opening fire. Hide Caption 44 of 45 Photos: Colorado movie theater massacre What is believed to be the suspect's car is examined after the shooting. Hide Caption 45 of 45

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The receipt showed that Holmes bought an $106.99 Blackhawk urban assault vest, a $52.99 Blackhawk Omega Elite triple pistol magazine, a $52.99 Blackhawk Omega Elite M16 magazine pouch, and a $77.99 Blackhawk Be-Wharned silver knife.

With shipping costs, the total bill came to $306.99, according to a copy of the receipt provided to CNN.

The firm sells equipment to military and police personnel -- as well as weekend warriors, Weinman said.

The gear that the firm believes it sold to Holmes is manufactured by a company called Blackhawk Company, a "popular brand in tactical circles," Weinman said.

"We were pretty shocked to have discovered it," Weinman said.

"Oh, my God, we couldn't believe it" was how one of the company's owners reacted, Weinman said.

Holmes paid for a two-day air delivery when he placed the order on July 2, which would seem to indicate he wanted the materials in a hurry, Weinman said.

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:

-- AR-15 rifle: A rifle that commonly fires bullets .223 inches 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.

AR-15 rifles come in many different, customizable forms.

Aurora Police Chief Oates said Friday night that investigators had determined the suspect bought more than 3,000 rounds of ammunition for the rifle. As for the 100-round rifle drum and the weapon's firing capability, Oates said he did not know whether the rifle was fully automatic or sem-automatic but "even if it was semi-automatic, I'm told by experts that with that drum magazine, he could've gotten off 50 to 60 rounds ... within one minute."

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

A 12-gauge 870 Remington shotgun.

Oates said the suspect had purchased 300 rounds for the shotgun.

-- .40-caliber Glock handgun: Common weapon for police. Similar in size to other popular handguns.

Oates said some 3,000 rounds of ammunition for the two Glocks had been bought through online purchases.

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.