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Police have arrested an American man who allegedly smuggled parts for as many as 2,000 grenades into Mexico for drug cartels, more than a year after he was released by U.S. agents who stopped him at the Arizona-Mexico border for having more than a hundred grenade parts.

While a Department of Justice official said the case will be added to the investigation of Operation Fast and Furious, a flawed effort by the Bureau of Alcohol, Tobacco, Firearms and Explosives to target gun-trafficking networks on the Mexican border, there is a twist: in the Jean Batiste Kingery case, the U.S. Attorney's Office, not ATF, appeared to be responsible for letting him walk.

In a letter to congressional committees investigating Fast and Furious, Luciano Cerasi, general counsel for the Federal Law Enforcement Officers Association, said Kingery is believed to have manufactured more than 2,000 hand grenades for the Sinaloan and La Familia Michoacana cartels.

In the letter to the U.S. Senate Committee on the Judiciary and the House Committee on Oversight and Government Reform obtained by The Associated Press, Cerasi said Kingery was detained on June 15, 2010 at the San Luis port of entry on the southwestern Arizona-Mexico border after more than 100 disassembled grenades were found hidden in a spare tire in his vehicle.

The next day, "the suspect confessed to his involvement in arming these cartels with hand grenades," teaching cartels how to convert AK-47 and AR-15 rifles into automatic weapons and to sometimes delivering instructions, including assassination orders, to cartel operatives.

An ATF agent "practically begged" a prosecutor at the U.S. Attorney's Office in Phoenix for permission to arrest Kingery but was denied, according to Cerasi, who wrote that the agent "was horrified with the thought of releasing this individual, who, in his opinion, was engaged in terrorist-like activity." Cerasi's group is a professional association, representing exclusively federal law enforcement officers.

But a lead prosecutor for the U.S. Attorney's Office wasn't interested in pressing charges because he viewed the grenade parts as novelty items that lacked jury appeal and instructed the ATF to let Kingery go, saying they could always file an indictment later, according to a law enforcement official close to the investigation who was not authorized to be quoted by name.

The indictment never came, although a different prosecutor for the U.S. Attorney's Office had started working on the case in May.

The official said Kingery was born in Illinois, and had been splitting his time between Arizona, southern California and Mazatlan, and had been working for the cartels for a couple of years. A database search showed that Kingery's last address in the U.S. was in Yuma, Ariz., and that he previously lived in San Diego.

Kingery had been on ATF's radar screen since January of 2009 when he bought multiple AK-47s at a Tucson gun shop. Online grenade parts sellers reported that Kingery was buying from them in large numbers in October of that same year.

The letter said the U.S. Attorney's Office prosecutor who declined to pursue the grenade-parts case or approve Kingery's arrest was Emory Hurley, a prosecutor who played a role in Fast and Furious and who was reassigned from criminal cases to civil case work in August.

The U.S. Attorney's Office in Phoenix deferred comment to Washington, and Hurley did not respond to an email request for comment. ATF spokesman Drew Wade said "we can't confirm or deny the existence of an ongoing criminal investigation."

Kingery's arrest has provided details on a network that allegedly supplied hundreds of hand grenades to Mexico's powerful Sinaloa cartel. Such grenades have been blamed in the injuries or deaths of dozens of civilians in Mexico, where grenades have been tossed into public squares, streets, bars and nightclubs.

In a separate controversial case, a congressional investigation says ATF agents of lost track of about 1,400 of the more than 2,000 guns whose purchase they had watched as part of Operation Fast and Furious.

In Washington D.C., Justice Department spokeswoman Tracy Schmaler said the department's inspector general has expanded that investigation to include the Kingery case.

"The department is aware of concerns raised" about the Kingery case "and has been looking into it," said Schmaler. "We have notified Congress about this operation and offered to brief them on it."

Mexico's Attorney General's Office said Kingery had been the subject of "a bilateral investigation" between Mexico, the U.S. government and the ATF.

The office said Kingery, 40, allegedly bought weapons parts and grenade casings in U.S. stores and even over the Internet, and smuggled them into Mexico through the border city of Mexicali.

The office said Kingery was arrested late last week in the Pacific Coast city of Mazatlan, in Sinaloa state, in a raid on a house where five guns were found. He is being held under a form of house arrest.

Police also raided five other homes, and found what appeared to have been facilities for assembling grenades, including gunpowder and grenade triggers, pins and caps.

In April, two men were arrested with 192 grenade casings in Baja California, the state where Mexicali is located. They told police they were part of the grenade smuggling ring, and that led to the detention of another American man, who led police to Kingery, prosecutors said.

The U.S. Embassy in Mexico City would not confirm the man's name, hometown or nationality, citing privacy concerns.

Mexican drug cartels have frequently used hand grenades in battles with police and soldiers, and occasionally against civilians.

A cartel-related grenade attack in the western city of Morelia killed eight people during the 2008 independence celebrations, when suspects tossed grenades into the city's crowded main square.

On Aug. 14, gunmen tossed a grenade onto a busy tourist boulevard in the Gulf Coast city of Veracruz, killing a man and seriously wounding his wife and their two young children.

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Associated Press writer Pete Yost in Washington and Mark Stevenson and Adriana Gomez Licon in Mexico City contributed to this story.