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.
In a report released Tuesday, the panel's Democratic minority said no evidence was found that Justice Department bosses in Washington "conceived or directed" the plan. Their report places blame for the widely criticized investigation on federal agents and prosecutors in Arizona.
While there is no evidence that the U.S. attorney's office in Phoenix or agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives "acted with anything but a sincere intent to stem illegal firearms trafficking," they "created an obvious and inexcusable threat to public safety on both sides of the border" by allowing weapons to reach the cartels, the Democrats concluded.
Republican Sen. Charles Grassley, one of the leading critics of the operation, dismissed the findings as "a knee-jerk defense (of the DoJ) from their political allies in Congress." And the Oversight Committee's chairman, California GOP Rep. Darrell Issa, released a letter to Holder on Tuesday in which he repeats demands for Justice Department documents he says are needed to determine "to what extent" the department "has obstructed our work."
"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," Issa wrote.
A Justice Department official, speaking on condition of anonymity, told CNN late Tuesday that the department "has continuously worked with the committee in its requests for information."
Issa has hammered away at Holder during the course of the committee's inquiry and accused Justice Department officials of misleading Congress; Holder has slapped back by calling GOP allegations about the program "inflammatory and inappropriate."
The operation came to light after the December 2010 killing of Border Patrol Agent Brian Terry, when two AK-47 variants that were allowed to "walk" under the program were found at the scene. It drew sharp criticism from top officials, who have long said U.S. weapons are fueling Mexico's bloody drug war.
The Justice Department initially denied that the ATF had let weapons cross the border, only to retract that statement as evidence of the botched probe mounted. Issa is calling for the Justice Department produce all related documents after that February 2011 letter.
But Tuesday's Democratic report states that Issa's committee "has obtained no evidence indicating that the attorney general authorized gun-walking or that he was aware of such allegations before they became public."
"None of the 22 witnesses interviewed by the committee claims to have spoken with the attorney general about the specific tactics employed in Operation Fast and Furious prior to the public controversy," the Democrats wrote.
Justice Department records released Friday night showed that one of Holder's top lieutenants, Criminal Division chief Lanny Breuer, and then-ATF Acting Director Kenneth Melson discussed whether illegal weapons could be intercepted in Mexico if U.S. and Mexican authorities worked together. Grassley pointed to that document Monday as evidence that Breuer knew about a gun-walking investigation as early as April 2010 and "stood mute" as congressional investigators demanded records from the department.
"They ignored the warning signs and failed to put a stop to it or hold anyone accountable," Grassley said in a statement on the Democratic report.
The Justice Department's independent inspector-general is conducting its own investigation of Fast and Furious, and President Barack Obama told ABC News in October that "people who screwed up will be held accountable."