Washington (CNN) -- Democrats say a House committee has found no evidence showing that top Justice Department officials were behind a gun-trafficking investigation that let hundreds of high-powered weapons reach Mexican drug cartels. The Republican head of the committee, meanwhile, accused the department of obstructing its probe of the botched operation and threatened Attorney General Eric Holder with contempt of Congress. Holder is expected to appear before the House Government Oversight and Reform Committee on Thursday. The panel has led a high-profile investigation into "Operation Fast and Furious," a gun probe run by federal agents in Arizona. It was one of several Phoenix-based operations intended to track the flow of illegally purchased American guns to the Mexican cartels -- but in practice, agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives allowed so-called straw buyers to take weapons across the border without being intercepted.
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NEW: Issa threatens Holder with contempt over gun probe
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A Democratic report blames agents in Arizona for Fast and Furious
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A top Republican critic calls it "a knee-jerk defense" of Washington allies
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The botched investigation allowed hundreds of guns to reach Mexican drug gangs
"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 @placeholder," Issa wrote.