“YOU don't lose guns. You don't walk guns. You don't let guns get out of your sight,” said Carlos Canino of the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), in an angry interview with congressional investigators. He was talking about Operation Fast and Furious, a fatally misconceived effort to fight drug-trafficking that had led some of his colleagues to lose guns deliberately.

The operation, outlined in two congressional reports last summer, began in 2009 in the Phoenix, Arizona, field office of the ATF, which is under the Department of Justice. The department was trying to be more active in Mexico's fight against its drug gangs, and decided that agents would allow known “straw purchasers” to buy guns from American shops. The straw buyers, the ATF reasoned, would bring the guns to the gangs. When the guns turned up again, the agents might be able to use them as evidence to build bigger cases.

A similar effort, Operation Wide Receiver, which ran from 2006 to 2007, was shut down because it was ill-conceived and dangerous. When the idea was revived many field agents in Phoenix were appalled. There might have been more resistance, but the operation was hidden from other border policing agencies, and even from ATF agents in other offices.

The critics got little attention until December 2010, when Brian Terry, a Border Patrol agent, was shot dead during a firefight. Two rifles recovered at the scene were part of Operation Fast and Furious. Whistleblowers started to come forward. In January 2011 William Newell, the special agent then in charge of the Phoenix office, announced 20 indictments. Most were of straw buyers who could have been indicted much earlier.

The battle over Operation Fast and Furious has only become more serious since then. There are more troubling questions about how much senior officials in the Justice Department, up to and including Eric Holder, the attorney-general, knew about the operation, and when. Last February the department issued a letter denying the allegations that the ATF had allowed gunwalking. In March Barack Obama told Univision that neither he nor Mr Holder had authorised the operation. Six weeks later, in May, Mr Holder told the House Judiciary Committee that he had “probably” first heard of Operation Fast and Furious “over the past few weeks”.

Explore our interactive map of Mexico's drug traffic routes, "cartel" areas and crime-related homicides

Last month the department withdrew its February letter, saying it was not correct. Testifying again a few days later, Mr Holder was sanguine when asked to clarify the difference between lying to Congress and misleading it: “Well, if you want to have this legal conversation, it all has to do with your state of mind.” He added that the department would not be turning over any materials related to the operation from later than February. But he is being hauled in to give more testimony next month.

Whatever happens, he will not be the only victim. Over the course of the operation some 2,000 guns were sold. Several hundred have since been recovered at dozens of crime scenes in the United States and Mexico. The rest are lost. The grim thing is that many of them will someday be found.