On Feb. 9, Cho Seung Hui walked into a pawnshop on Main Street in Blacksburg, directly across the street from the Virginia Tech campus, and picked up one of the guns he would use in his deadly rampage Monday: a Walther .22-caliber pistol, a relatively inexpensive firearm most commonly used for target shooting or plinking cans.

One month later, on March 16, Cho stepped into Roanoke Firearms, a 3,000-square-foot, full-service gun dealer where more than 350 guns are on display. Cho offered his driver's license, a checkbook that showed a matching address and an immigration card.

Once an instant background check confirmed his clean criminal record, Cho had little else to do, other than pay $571, to become the legal owner of a Glock 19 and a box of 50 cartridges.

(By April Umminger and Cristina Rivero /The Washington Post)

With those two handguns -- both easy to use, reliable and semiautomatic -- Cho, 23, carried out a shooting rampage that left 33 people dead, including himself, and injured nearly as many.

Cho's choice of weapons and ammunition explained how he could kill and injure so many people so quickly.

The Glock, often carried by police and members of the military, is also a popular choice for civilians interested in self-defense, gun experts say. Once the trigger is depressed and the bullet fired, the gun ejects the empty shell casing, chambers a new round and is ready to shoot again immediately. The .22-caliber pistol operates in a similar fashion.

Cho's Roanoke purchase, captured on the store's video surveillance, was unremarkable. The owner described Cho as low-key and clean-cut.

"He filled out the paperwork. I sent it to the state police. They gave him a clean bill of health," said owner John Markell. "We're very careful about screening people. We size people up all the time. If we think they're fidgety, we won't sell them a gun."

Joe Dowdy, owner of JND Pawnbrokers in Blacksburg, said Cho did not purchase the gun from him but came into his shop to pick it up, probably after buying it on the Internet. Dowdy said he received the gun from another vendor. Cho came into the shop, showed his ID, filled out some paperwork, waited for a background check and paid a $30 fee.

"People are saying I sold him the firearm," Dowdy said. "I did not."

Dowdy said he cannot be sure that Cho purchased the gun online but that is the most likely explanation for why another vendor would have sent it to the pawnshop for Cho.

Both transactions were legal. Unlike some other states, Virginia has no waiting period before purchasing a handgun; nor does it require registration. State law does limit purchasers to one gun per month.

Law enforcement officials say the Glock that Cho used had a 15-round magazine, illegal under the federal assault weapons ban that expired in 2004. Several empty magazines were recovered at the scene.

"If you have four or five rounds in a clip rather than 15, the shooter has to reload and reload," said Brian Malte, with the Brady Campaign, a gun safety group. "That gives someone an opportunity to do something to stop him."

A customer enters Roanoke Firearms in Roanoke yesterday, where shooter Cho Seung Hui bought a weapon.
(Chip Somodevilla/Getty Images)

Law enforcement officials have tied the 9mm Glock to both the shootings at a residence hall early Monday morning and to the massacre at Norris Hall two and a half hours later. Investigators were easily able to trace the Glock to Roanoke Firearms in Roanoke because the receipt for the gun was in the backpack Cho carried Monday.

Officials from the Bureau of Alcohol, Tobacco, Firearms and Explosives hand-delivered the two handguns, shell casings and bullet fragments found at the crime scenes to the ATF National Laboratory outside of Beltsville, according to ATF Special Agent Rich Marianos. There, investigators worked through the night to "raise" or reveal the serial numbers on the guns, which had been obliterated, so they could be traced to their manufacturer, distributor and then gun shops.

Tool mark examiners were able to match the bullet fragments and casings to the firearms through a process sometimes called "ballistics fingerprinting."

When a bullet is fired, it is rotated in a gun barrel like a spiraled football. The passage through the barrel marks the sides of the bullet, leaving a pattern of alternating raised lines called "lands" and gouged lines called "grooves." These patterns and even more minute impressions, which can be examined and measured under a microscope, are unique in every firearm.

An individual weapon leaves the same distinct marks on every bullet fired, allowing recovered bullets to be matched to a particular gun. The guns are also test-fired into a tank of water and the bullets recovered to be compared with bullets from crime scenes.

By yesterday morning, the ATF had provided the Virginia State Police with all the information matching the bullets to the guns.