WASHINGTON - The Department of Justice reassigned the acting head of the Bureau of Alcohol, Tobacco, Firearms and Explosives to a new post Tuesday after months of investigation into the agency's Fast and Furious gun-tracking operation.

Kenneth Melson, who had served as ATF's acting director since 2009, will become a senior adviser for forensic science in the Justice Department's Office of Legal Policy on Aug. 31, the department said in a statement.

Also on Tuesday, Arizona's U.S. attorney, Dennis Burke, resigned.

Burke had overseen Operation Fast and Furious, a joint federal law enforcement effort to take down Mexican cartels importing U.S.-purchased weapons and shipping drugs to the United States.

ATF agents in Phoenix were instructed to watch sales of firearms to straw purchasers and track the weapons instead of interdicting them, in a practice known as "gunwalking."

The aim was to dismantle the cartels' entire structure instead of picking off low-level operatives. But the government lost track of 1,000 or more guns, and two of them were recovered in December from the scene of a U.S. Border Patrol agent's murder in Arizona. A report released in late July by the House Oversight and Government Reform Committee alleged that some guns were later recovered from other crime scenes in Mexico.

The committee is conducting a broad investigation of Operation Fast and Furious. The Justice Department's inspector general also is investigating the operation. ATF is a subordinate unit of the Justice Department. No release date has been set for the IG report.

The Justice Department appointed B. Todd Jones, currently U.S. attorney in Minnesota, to replace Melson as acting ATF director.

Jones will stay in his current role while serving as ATF head and will continue living in Minnesota, said Tracy Schmaler, a department spokeswoman.

Reaction to the shake-up was itself fast and furious.

"Instead of reassigning those responsible for 'Fast and Furious' within the Department of Justice, Attorney General (Eric) Holder should ask for their resignations and come clean on all alleged gun-walking operations, Sen. John Cornyn, R-Texas, a member of the Senate Judiciary Committee, said in a statement.

Rep. Darrell Issa, R-Calif., chairman of the government reform committee, said in a statement that he still has questions about the operation. Even so, he said, "these changes are warranted and offer an opportunity for the Justice Department to explain the role other officials and offices played in the infamous efforts to allow weapons to flow to Mexican drug cartels."

Dan Freedman of the Washington bureau also contributed to this report.

Puneet@hearstdc.com