Virginia Tech university gunman Cho Seung-Hui walked into a gun shop five weeks ago, put down a credit card and walked out with a Glock 19 handgun and a box of ammunition. He paid $571 (€420).
The Glock was one of two guns found with his fingerprints after he shot dead 32 people and then himself in the deadliest shooting rampage in modern US history.
Roanoke Firearms owner John Markell said his shop sold the Glock to Cho in March. The serial number had been scratched off, but federal agents traced it to the store using a receipt found in Cho's backpack.
"It was a very unremarkable sale," said Mr Markell, who did not handle the sale personally. "He was a nice, clean-cut college kid. We won't sell a gun if we have any idea at all that a purchase is suspicious."
Markell said it was not unusual for college students to make purchases at his shop as long as they were old enough. "To find out the gun came from my shop is just terrible," Mr Markell said.
Cho was a legal, permanent resident, according to federal officials. He had lived in the US since 1992. That meant he was eligible to buy a handgun unless he had been convicted of a felony.
Authorities also found a Walther .22 calibre handgun in Cho's possession, according to a search warrant filed.
Because he killed and injured so many victims in a short span of time, some people speculated that Cho used high-capacity magazines containing as many as 33 rounds in each clip.
Under the federal assault-weapons ban enacted in 1994, magazines were limited to 10 rounds. But that ban was allowed to expire in 2004.
"The key thing that we have seen in all of these school shootings is easy access to high firepower weapons," said Daniel Vice, a lawyer with the Brady Campaign to Prevent Gun Violence. "These killings can't be done with baseball bats and knives."
Under Virginia law, state police keep records of gun purchases from licensed dealers for only 30 days. After that, police destroy the records.
Fairfax County spokeswoman Merni Fitzgerald said there was no record of Cho applying for a concealed-weapon permit there.