Washington (CNN) -- Firings and charges against Justice Department officials who oversaw the agency's flawed gun-running operation are likely to come in the next six months, Attorney General Eric Holder said Thursday.
Holder was speaking before the House Oversight and Government Reform Committee on the controversial sting operation called Operation Fast and Furious.
Asked what steps he had taken since the controversy came to light 13 months ago, Holder said he is awaiting the conclusions of an internal investigation.
Regarding people found culpable in that inquiry, he said, "I can assure you those people will be removed from federal service."
Any possible charges will not be filed until the department has the evidence to get a conviction, he said at the hearing.
The operation, designed to monitor the flow of weapons, allowed illegally purchased firearms to be taken from U.S. gun stores across the Mexican border to drug cartels.
However, hundreds of weapons were lost or unaccounted for, and a storm of outrage erupted when two of the missing weapons were found at a site where Border Patrol agent Brian Terry was killed in December 2010.
Terry's family filed a $25 million wrongful death claim Wednesday against the federal government, charging that U.S. investigators allowed murder weapons into the hands of criminals.
"The murder of Agent Terry and other acts of violent crimes were the natural consequence of (the) decision to let dangerous weapons designed to kill human beings 'walk' into the hands of violent drug-trafficking gangs," the claim said.
Democrats and Republicans have been at odds over who knew what about the operation and when.
At the outset of the hearing, new information on the operation was scarce, and the back-and-forth between members of the two parties dominated.
Rep. Darrell Issa, R-California, threatened to begin a contempt proceeding against Holder to force him to release more documents and pressed him for proof that such gun-running will not happen again.
Republicans are investigating how high up the operation was sanctioned, while Democrats defended Holder's testimony that he did not know about it.
The showdown comes after Issa and the Justice Department exchanged sharp letters in recent days.
The Justice Department sent a pointed letter to Issa late Wednesday, rejecting his demand on Tuesday that certain documents be turned over by next week.
In his letter to Holder, Issa blasted the Justice Department for dumping -- after the close of business Friday -- important, long-sought documents that disclosed that Assistant Attorney General Lanny Breuer had discussed with Mexican officials a plan to allow illegal guns "to walk" into Mexico, where Mexican officials would arrest the gun-runners.
Issa said an e-mail showed that "Breuer suggested allowing straw purchasers cross in Mexico so (Mexican authorities) can arrest, prosecute and convict."
"These new documents show that Breuer made this statement on February 4, 2011, the same day Assistant Attorney General Ronald Weich wrote to Congress denying that the department allowed guns to walk," Issa said.
The Justice Department fired back Wednesday, claiming that Issa's letter showed "significant misunderstanding both of the documents we recently produced and of the Department's position on the issues you raise."
The five-page letter signed by Deputy Attorney General James Cole strongly rejected the claim that the documents show that Breuer supported letting illegal guns make their way to Mexican cartels.
"Assistant Attorney General Breuer proposed to his Mexican counterparts a scenario in which those carrying illegal weapons across the border would be arrested at the border by Mexican officials and charged in Mexico. While these officials ultimately did not pursue that strategy, it is neither fair nor accurate to say that this was advocacy of 'gunwalking'. It was not."
Republicans have pressed to learn whether top Obama administration officials were aware of the controversial tactics, but to date, Breuer is the highest-ranking official shown to have any awareness of any possible gun-running operation.
Democrats have released documents suggesting that the U.S. attorney's office in Phoenix and the Bureau of Alcohol, Tobacco, Firearms and Explosives field office in Phoenix were at the heart of the decision-making in developing sting operations, including Operation Fast and Furious.
Issa said in his letter to Holder that, "if the department continues to obstruct the congressional inquiry by not providing documents and information, this committee will have no alternative but to move forward with proceedings to hold you in contempt of Congress."
The Justice Department response Wednesday rejected Issa's February 9 deadline to produce all demanded documents. Cole called the deadline "impossible" to meet because of the broad scope of the request.
He did not directly refer to the threat of a contempt charge by Issa.