Weapons Purchases Aroused No Suspicion

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."

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.