How did a gun belonging to a former assistant special agent in charge at the Bureau of Alcohol, Tobacco, Firearms and Explosives end up at a crime scene in Mexico where five died, including a Mexican beauty queen?

That's the question being asked by congressional investigators and ATF officials in Phoenix.

A FN Five-Seven semi-automatic pistol, a high powered handgun originally

restricted to military and law enforcement customers, was recovered by Mexican police at the scene of a Nov. 23 shootout between the Sinaloa Cartel and the Mexican military.

Records show the gun was purchased in January 2010 by George Gillett, the former No. 2 in the ATF office in Phoenix. Gillett now works at ATF headquarters in Washington as a liaison to the federal Bureau of Prisons.

Gillett purchased the weapon at Legendary Arms, a Phoenix gun store. On the federal form 4473 used to buy the gun, Gillett used the ATF office address, 201 East Washington, and said "Apt 940." On subsequent purchase, Gillett used a commercial address, that of a strip mall.

Both actions are illegal, since ATF regulations require buyers use their residential address.

"Lying on form 4473 is a felony and can be punished by up to five years in prison in addition to fines," Sen. Charles Grassley said in a letter Wednesday to Michael Horowitz in the Office of Inspector General. "I request that you initiate an investigation into these matters and that you specifically examine whether Mr. Gillett was the purchaser as indicated by these documents, why the forms list multiple, inaccurate residential addresses while purchasing the weapons, and how the weapon purchased on January 7, 2010 ended up in Mexico."

Gillett's gun was found in Sinaloa after a gun battle that killed Mexican beauty queen Maria Susana Gamez. Gamez was reportedly fighting alongside the cartel. Police found a weapon similar to an AK-47 and some 50 bullets next to her body. Another weapon found at the crime scene was traced to a Uriel Patino, who illegally bought more than 600 guns and is a main suspect in the controversial Operation Fast and Furious run out of the Phoenix office of the ATF.

"Why would the assistant in charge of that office be buying guns in the first place?" Grassley said in an interview with Fox News. That would raise the question of the extent to which that person might be involved in the gun trafficking that was going on and profiting from it. ... These are legitimate questions."

Gillett didn't respond to a request to comment for this story, though he has confirmed in other news interviews that he bought the gun, saying he later sold it on the Internet.