Bureau of Alcohol, Tobacco, Firearms and Explosives agents have connected a gun at a hideout of Mexico's most wanted drug lord, Joaquín "El Chapo" Guzmán, with a flawed U.S. operation. (AP)

Bureau of Alcohol, Tobacco, Firearms and Explosives agents have connected a rifle recovered at a hideout of Mexican drug kingpin Joaquín “El Chapo” Guzmán with a flawed U.S. operation that allowed illegally purchased guns to flow into the hands of narcotics traffickers, the Justice Department confirmed.

In a written response to a congressional request, Assistant Attorney General Peter J. Kadzik wrote that a .50-caliber rifle found at the Los Mochis residence Guzmán used shortly before his capture was found to be associated with the “Fast and Furious” operation in which ATF agents did not immediately arrest illegal gun buyers and sellers so they could track the flow of the weapons to Mexican drug traffickers.

The rifle, Kadzik wrote, was purchased July 8, 2010, by someone the ATF did not know but who later was identified and “became a subject” of the operation. Kadzik wrote that the person, whom he did not name, was never indicted. Although the rifle’s initial purchase and transfer were illegal, he wrote, the ATF had not connected the weapon to other crimes.

The revelation — which was reported in January by Fox News — demonstrates the continued fallout from an operation that has been widely condemned. The operation allowed more than 2,000 weapons to hit the streets, including two used in the 2010 killing of a Border Patrol agent. It sparked a congressional inquiry and led to the ouster of the U.S. attorney in Phoenix who approved it.

Kadzik wrote that the rifle was one of 19 guns recovered from the residence that Mexican military forces raided in January in an attempt to take custody of Guzmán. He did not indicate whether the weapon was fired during the incident.

Guzmán had escaped a Mexican prison and was on the run. Five of his associates were killed and a Marine was injured in the gun battle in Los Mochis, and Guzmán was captured about five miles away after fleeing the residence through a sewer.

A Justice Department spokesman declined to comment. Kadzik’s letter came in response to a request from U.S. Rep. Jason Chaffetz (R-Utah), who chairs the House Oversight and Government Reform Committee, and Sen. Charles E. Grassley (R-Iowa), who chairs the Judiciary Committee, both of whom asked about the rifle.

Kadzik also described four other guns in which Fast and Furious weapons had some tie to violent incidents.

Two guns associated with the operation were recovered after a May 22 raid on a ranch in Michoacan, Mexico, controlled by the Cartel de Jalisco Nueva Generacion, Kadzik wrote. The raid led to a lengthy gun battle that killed 42 suspects and one police officer, although it was unclear whether the Fast and Furious guns were fired, Kadzik wrote.

Another Fast and Furious gun was identified as having been used during the killing of three Mexican municipal police officers in July, although it was not clear whether the weapon fired a fatal shot, Kadzik wrote. That weapon was recovered from another gruesome scene in which three people — suspects in the killings of the officers — were found dead in or around an abandoned vehicle in Chihuahua, Mexico, Kadzik wrote.

A fourth Fast and Furious gun was recovered in the vehicle of a suspect in an armed robbery in Sonora, Mexico — although it was unclear whether the firearm was used or fired during the robbery, Kadzik wrote.

Kadzik wrote that as of Jan. 28, 885 guns purchased by Fast and Furious targets had been recovered: 415 in the United States and 470 in Mexico.

“ATF and the Department deeply regret that firearms associated with Operation Fast and Furious have been used by criminals in the commission of violent crimes, particularly crimes resulting in the death of civilians and law enforcement officers,” Kadzik wrote. “ATF accepts full responsibility for the flawed execution of Fast and Furious, and will continue to support Mexican law enforcement in efforts to recover and identify associated firearms.”

Guzmán remains in custody in a maximum-security prison, although one of his lawyers has said he is willing to plead guilty to U.S. charges if he can be housed in a medium-security prison. Several U.S. attorney’s offices have criminal cases against him.