WASHINGTON -- A 5.7-millimeter pistol used in the Fort Hood shooting was purchased legally by suspect Nidal Hasan at a Texas gun shop, law enforcement officials said Friday.

The officials spoke on condition of anonymity because they were not authorized to discuss the case.

Records indicate Hasan bought the FN 5.7 at store called "Guns Galore" in Killeen, Texas, in recent months and that gun was used in the attack that left 13 people dead, one of the officials said. The pistol has been dubbed a "cop killer" by those who have tried to stop its use.

Army officials said Hasan also was carrying another handgun.

One of the law enforcement officials said that gun was an older model Smith & Wesson .357-caliber revolver, but the official added there's nothing so far to indicate the second weapon was fired. The official said the revolver was so old, investigators doubt it could be traced to a specific purchase.

The most powerful type of ammunition for the FN 5.7 gun is available only to law enforcement and military personnel. Gun control advocates call it a "cop killer" weapon because that ammo can pierce bulletproof vests, and its use by Mexican drug cartels worries police.

It is not clear what kind of ammunition was used in the Fort Hood attack.

Agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives, as well as the Texas Rangers have interviewed store personnel about Hasan's gun purchase. One official said the store records do not indicate Hasan purchased any ammo when he bought the gun.