(CNN)Here's a look at Operation Fast and Furious. From 2009-2011, the Phoenix Field Division of the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), along with other partners, allowed illegal gun sales in order to track the sellers and purchasers, who were believed to be connected to Mexican drug cartels.
Operation Fast and Furious Fast Facts
During the Fast and Furious investigation, nearly 2,000 firearms were illegally purchased for $1.5 million, according to a DOJ inspector General report. Hundreds of guns were later recovered in the United States and Mexico.
In 2010, two of the weapons linked to Fast and Furious turned up near the scene of the murder of Border Patrol Agent Brian Terry in the Arizona desert.
Whistleblowing led to investigations by the Senate Judiciary Committee and the House Oversight and Government Reform Committee. During the House investigation, Attorney General Eric Holder was cited for contempt.
Fast and Furious was one of the operations under Project Gunrunner, part of the Department of Justice's broader National Southwest Border Counternarcotics Strategy.
Operation Fast and Furious was not the ATF's first "gun walking" investigation, which allowed illegally purchased firearms to "walk" out of gun shops. It was preceded by Operation Wide Receiver, which began in 2006.
October 31, 2009 - Phoenix-based ATF agents get a tip from a local gun shop about suspicious purchases of assault rifles by four individuals. The agents begin investigating whether the individuals were "straw purchasers" working for a large-scale illegal gun trafficking organization. The probe later comes to be known as Operation Fast and Furious.
December 14, 2010 - Terry is killed during a shootout in the Arizona desert. Five of the men involved in the shootout are later convicted of murder. One associate is later convicted on conspiracy and firearms offenses in connection with the killing. The last suspect is arrested in 2017 and is awaiting trial.
January 25, 2011 - The US Attorney's office in Arizona announces that 34 suspects have been indicted for firearms trafficking from the United States to Mexico. The DOJ inspector general later reports that 20 of the defendants were caught via Fast and Furious.
January 27, 2011 - Senator Chuck Grassley of the Senate Judiciary Committee sends a letter to Kenneth Melson, acting director of the ATF, regarding the agency's firearms trafficking investigation and allegations that the ATF allowed hundreds of assault weapons to be smuggled to Mexico. He notes that two of the guns may have been used by Terry's killers.
March 3, 2011 - Melson announces the formation of a panel to "review the bureau's current firearms trafficking strategies employed by field division managers and special agents."
April 1, 2011 - The House Oversight Committee issues a subpoena for ATF documents.
May 3, 2011 - Holder testifies before the House Judiciary Committee. He says that he first heard of Fast and Furious only over the past few weeks.
June 2011 - Whistleblowers testify on Capitol Hill.