By David Schwartz

PHOENIX (Reuters) - Federal authorities were probing how a gun bought by a former top U.S. federal agent in Phoenix ended up at the scene of a fatal shootout that killed a beauty queen in Mexico, a U.S. law enforcement official said on Wednesday.

The weapon, found after a face-off between members of the Sinaloa drug cartel and Mexican military police, had been originally purchased by former Alcohol, Tobacco, Firearms and Explosives Assistant Special Agent in Charge George Gillett, said the U.S. official, who declined to be named.

Killed in the deadly gun battle was Mexican beauty queen Maria Susana Flores Gamez and four others. The U.S. Justice Department Office of the Inspector General was investigating, the official said.

Gillett, who could not be reached for comment, was a key supervisor in a botched ATF sting known as "Fast and Furious," a controversial operation that allowed more than 2,000 weapons to slip across the U.S. border into Mexico.

The bungled operation, triggered by gun purchases made in the Phoenix area, was envisioned as a way to track weapons from the buyers to senior drug cartel members.

Federal agents who ran the operation focused on building cases against the leaders of a trafficking ring, and did not pursue low-level buyers of those firearms.

Two of those guns were discovered at the murder scene of U.S. Border Patrol Agent Brian Terry, killed in a shoot-out with suspected border bandits in southern Arizona in December 2010. It was not clear if the fatal bullets came from the weapons.

The gun found at the November crime scene in Mexico was an FN Herstal pistol, a high-powered weapon favored by Mexican drug cartel members, according to the law enforcement official. Continued...