(CNN) Here's some background information about 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.

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 where US Border Patrol Agent Brian Terry was gunned down 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.

December 14, 2010 - Four of the men involved in the shootout are later convicted of murder. Two other associates are later charged with conspiracy and firearms offenses in connection with the killing. The last suspect is arrested October 2017. Terry is killed during a shootout in the Arizona desert. Four of the men involved in the shootout are later convicted of murder. Two other associates are later charged with conspiracy and firearms offenses in connection with the killing. The last suspect is arrested October 2017.

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 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.

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 had first heard of Fast and Furious only over the past few weeks.

June 2011 - Whistleblowers testify on Capitol Hill.

July 26, 2011 - The House Oversight Committee holds a second hearing.

October 12, 2011 - The House Oversight Committee issues a subpoena for communications from Holder and other DOJ officials related to the federal gunrunning operation.

June 12, 2012 - Holder rejects calls for his resignation from Republicans during a House Oversight Committee hearing.

June 28, 2012 - This is the first time in American history that the head of the Justice Department has been held in contempt by Congress. The House of Representatives votes 255-67 to hold Holder in criminal contempt of Congress. This is the first time in American history that the head of the Justice Department has been held in contempt by Congress.

July 6, 2012 - The White House and the DOJ announce that Holder will not face criminal prosecution under the contempt of Congress citation.

July 31, 2012 - The first of a three-part joint Congressional report is released. The report prompts The first of a three-part joint Congressional report is released. The report prompts the resignation of William Hoover, the deputy director of the ATF.