The scandals surrounding Operation Fast and Furious — a failed tactic in which the Bureau of Alcohol, Tobacco, Firearms and Explosives allowed thousands of illegally purchased guns "to walk" into Mexico — just won't go away.

The clumsy program that ran from late 2009 until early 2011 was intended to weaken Mexican cartels. Instead it did the exact opposite when investigators at the ATF lost track of about 2,000 of the weapons.

Dozens of the weapons have since resurfaced south of the border, in the hands of bloodthirsty criminals.

Now the ATF has revealed there was one of them among a small arsenal seized at the safe house used by Joaquín "El Chapo" Guzmán Loera, just before his arrest. Chapo escaped a Mexican navy raid on the house in the early hours of January 8 thanks, in part, to the fight put up by his gunmen, who presumably used the weapon. The world's most-wanted drug lord slipped out of the house via a tunnel and then stole a car. He was captured because of a stolen car report.

"It was a Barrett .50-caliber," ATF special agent Tom Mangan told VICE News on Friday from the agency's Phoenix office. He added that the firearm was one of 11 found at the house. "We know when, and where, as well as who purchased the gun."

The confirmation that a Fast and Furious gun was in the possession of gunmen who helped buy El Chapo time in the initial raid is particularly shocking, but this isn't the first time weapons the ATF lost track of during its reckless "gun-walking" operation have ended up in the hands of powerful cartels, or been discovered at a violent crime scene. And authorities agree it won't be the last.

"Those guns are going to be recovered for the next 20 years," Mangan said. "But some of them we'll never find, because they may never get traced by law enforcement in this country, or — given that so many ended up in Mexico — whether those departments will ever even trace those guns."

Republican Sen. Charles Grassley and Congressman Jason Chaffetz, who heads the House Oversight committee, sent a letter to Attorney General Loretta Lynch on Wednesday, demanding the serial numbers of every gun that remains unaccounted for.

Or, as Chaffetz said Thursday, "every weapon ATF sold to drug cartels."

As of a March 2013, the Department of Homeland Security reported that only 105 of the nearly 2,000 lost weapons had been recovered by the ATF investigators who were tasked with tracking them. Another 462 were found in the course of duty by police on both sides of the border, many at gruesome crime scenes. More than 1,400 remained unaccounted for.

Officials confirmed on Friday that these remain the most recent figures available to the public.

The ATF operation, which was shrouded in secrecy, exploded in the public eye after a Customs and Border Patrol agent, Brian Terry, was killed in a December 2010 shootout near the Mexican border in Arizona.

After his death, several ATF employees came forward to blow the whistle. Two of the weapons found at the scene traced back to Operation Fast and Furious.

Terry's family sued the US government and the case was thrown out in court, but his death drew attention to the flawed investigation and questions grew about the human cost of the ATF's efforts to acquire information about criminal gun-smuggling networks.

ATF data published last week shows that more than 70 percent of the firearms that enter Mexico, unsurprisingly, originate in the US. But citizens on both sides of the border were shocked to discover that the US government — through Operation Fast and Furious — had been helping to provide Mexico's most violent criminals with the tools with which to terrorize their victims.

"Obviously the Fast and Furious investigation was flawed. People were held accountable and punished for it," ATF agent Mangan said on Friday. "That type of investigation is not how ATF does its work."

But the discovery of the Fast and Furious gun at El Chapo's hideout is a reminder that the operation has a legacy.

'We know when, and where, as well as who purchased the gun.'

The scandal led to a drawn-out congressional inquiry, which began in 2011. ATF director Kenneth Melson, high-ranking justice department official Jason Weinstein, Arizona's US Attorney Dennis Burke, and his criminal division chief Patrick Cunningham, all resigned as a result. Congress voted to hold then Attorney General Eric Holder in contempt for his refusal to turn over documents requested, marking the first time in US history an active member of the president's cabinet has been hit with that sanction. Holder resigned in 2014.

A veteran ATF agent told oversight investigators from the Department of Justice that before the gun-running operations began agents had been "content to make the little cases," nabbing criminals just after they made their illegal purchases. But then ambitions grew, and the aim became to gather enough evidence to eventually unravel a major drug trade organization.

"Merely seizing firearms through interdiction will not stop firearms trafficking to Mexico," reads an October 2009 memo to border supervisors from the office of Deputy Attorney General David Ogden — who has since resigned. "We must identify, investigate, and eliminate the sources of illegally trafficked firearms, and the networks that transport them."

ATF investigators along the border apparently wildly misinterpreted the DOJ request, and Fast and Furious was born. Although large-scale gunwalking had been occurring under several different program names as early as 2006, the ATF's deputy attaché in Mexico, Carlos Canino, later testified that it was Operation Fast and Furious that led his counterparts in Arizona to "grab the ATF rulebook on [weapon] trafficking and throw it out the window."

"It's groupthink gone awry," Canino told congressional investigators at the time. "A perfect storm of idiocy."

The ATF had ceased to interdict weapons in the hope that they would lead agents to bigger targets. After observing — and facilitating — the illegal purchase and transfer of large weapon caches, agents would then inexplicably cease surveillance, losing track of the guns and allowing them "to walk."

Canino and his former ATF supervisor Darren Gil said the revelations were "so inconceivable" that they initially did not believe the whistleblowers when they came forward after agent Terry's death. This despite the fact that the two Mexico City-based agents had been the first to alert the Phoenix field division that an inordinate number of the weapons surfacing at violent Mexican crime scenes in 2009 traced back to their office's unending investigation.

In November 2009, Mexican authorities intercepted a .50-caliber rifle and 41 AK-47s in Naco, Sinaloa. The guns had walked out of Arizona less than 24 hours before — straight into the hands of the Sinaloa cartel, ATF records showed.

Less than three weeks later, police in the border city Mexicali seized 42 firearms from the Sinaloa cartel, along with nearly $3 million in cash. All of the weapons traced back to Fast and Furious.

The following month, another 40 weapons were seized in El Paso just before entering Mexico. They were on their way to — you guessed it — the Sinaloa cartel.

One ATF special agent then warned lead case agent Hope McAllister in an email that the Sinaloa cartel "is arming for a war." But the agency continued to withhold information from authorities and ATF agents in Mexico — who would later argue they'd been put in mortal jeopardy.

After detecting the "alarming number" of weapons being inexplicably traced back to the Phoenix office, the two ATF agents who were still unaware of the existence of Fast and Furious, Gil and Canino, began complaining to headquarters.

According to a 2011 congressional report, ATF leadership in both Phoenix and Washington repeatedly ignored the agents' concerns, and actively blocked them from sharing pertinent information with partners in Mexico.

By March 2010, ATF intelligence analysts finally briefed the Department of Justice that more than 1,000 guns had walked — and many had ended up in Mexico. Then ATF attaché in Mexico, Darren Gil, said his repeated attempts to have the program halted made work feel "like Groundhog Day."

Gil said his frustration and concern for the safety of his colleagues in Mexico culminated in "shouting matches" with supervisors, which proved fruitless.

Finally, that July, ATF headquarters told the Mexico-based agents that they would shut down the program. But they didn't.

When Gil left his position in October 2010, Fast and Furious continued. And its weapons continued to pop up all over the place.

The Chihuahua state attorney general's brother, Mario Gonzalez Rodriguez, was kidnapped by cartel members that month. The heavily armed group released a video of him being interrogated, before murdering him and dumping his body along the road to the border city of Ciudad Juarez. This led to a shootout with Mexican authorities who then seized more than a dozen weapons, two of which led back to the stateside ATF investigation.

Operation Fast and Furious finally ended in late January 2011, the month after CBP agent Terry's murder. Instead of achieving its goal of helping to take down a major drug cartel it had used American resources to further fuel bloodshed in Mexico and, it seemed, had even strengthened its target group.

Republican Senator Darrell Issa told Congress in 2011 that by allowing Mexican drug trade organizations to acquire AK-47s, AR-15s, .50 caliber sniper rifles, and other military-grade weaponry, an estimated 200 Mexicans had been killed.

ATF agents testified that this lapse empowered members of Mexico's most powerful drug trade organization, the Chapo-led Sinaloa cartel, in their fight against other groups in the country's brutal drug wars. Providing the cartel with weapons, agents said, had "changed the outcome of the battle."

Despite being over, the consequences of the operation continue to snowball.

In April 2011, Mexican authorities seized 40 Fast and Furious-linked weapons from the Juarez home of José Antonio Torres Marrufo, or "the Jaguar," who ran an armed wing of the Sinaloa cartel.

In November 2012, a 20-year-old Mexican beauty queen, María Susana Flores Gámez, was one of five people killed in a bloody gun battle. Authorities said members of the Sinaloa cartel had "used her as a human shield" during crossfire with Mexican soldiers.

An AK-47 recovered from the scene had been bought in March 2010 during Fast and Furious within a batch of 700 weapons purchased by a middle-man who was under ATF surveillance.

A different weapon found at the gruesome scene was personally purchased by ATF agent George Gillett. He bought the gun in January 2010 at a Phoenix gun store, using the address of the Phoenix ATF division on the forms. Gillett, who has since retired, oversaw Fast and Furious during its most disastrous months between 2009 and 2010 as the Phoenix division's second-in-command. There were no repercussions.

During this period, hundreds of lost firearms slipped into the hands of not only the Chapo-led Sinaloa cartel, but also an offshoot of the Beltrán Leyva cartel led by "El Teo," who went on to terrorize northern Mexico during the height of the drug war.

They have also been found in arsenals linked to the La Familia cartel in the western state of Michoacán. Several weapons seized after a major operation against the cartel in May 2011 — in which five government helicopters came under heavy fire — were traced back to guns that were supposed to be being monitored by Fast and Furious.

The gun that killed Jalisco police chief Luis Lucio Rosales Astorga in January 2013 was found within a cache attributed to the Gulf Cartel. It had been purchased in early 2010, more than 1,200 miles from the Lone Wolf Trading Company in Phoenix — a store that has now become synonymous with Fast and Furious.

'We ATF armed the cartel...It's disgusting'

Now the discovery of a Fast and Furious-linked weapon at El Chapo's last safe house — one capable of shooting down a helicopter — once again brings up the many unanswered questions left by the operation.

In an effort to shield the public from a cache of Department of Justice documents, which include correspondence between Eric Holder and his wife and mother, President Obama invoked executive privilege in June 2012. The first time he had exercised this privilege.

This Tuesday, however, a federal judge ruled that executive privilege does not apply in this case, in part because the privilege to conceal the documents is outweighed by the need for information. Judge Amy Berman ordered the administration to produce the documents by February 2, 2016.

Agent Canino, who was transferred from Mexico City to the Phoenix ATF office in 2012 to help clean up after the disastrous operation, said back in 2011 that there was a disturbing truth "nobody wants to talk about."

"[Agent] Terry is not the last guy," he told congressional investigators. "Unfortunately, there are hundreds of Brian Terrys probably in Mexico."

"We ATF armed the cartel," he added. "It's disgusting."