WASHINGTON — Top Justice Department officials largely dismissed a new House Republican report on the Fast and Furious gun-tracking operation, saying Tuesday that they have moved ahead with major reforms to prevent future incidents of U.S. firearms being lost and smuggled across the border into Mexico.

Further, the attorney for the ATF special agent in charge in Phoenix, who oversaw Fast and Furious, lashed out at Republican congressional leaders, calling their report a “political witch hunt” aimed less at finding fault than pinning Democrats with negative political fallout from Fast and Furious.

The report by Rep. Darrell Issa (R-Vista), chairman of the House Oversight and Government Reform Committee, and Sen. Charles E. Grassley, the top Republican on the Senate Judiciary Committee, concluded that five supervisors at the federalBureau of Alcohol, Tobacco, Firearms and Explosives were collectively responsible for Fast and Furious, which allowed about 2,500 weapons to be illegally purchased in Arizona.

But Tracy Schmaler, the top spokeswoman at the Justice Department, said the report was filled with “distortions and now-debunked conspiracy theories.” She did, however, credit the report for acknowledging that the idea for “gun walking” — allowing illegal weapons sales on the border — originated under the Republican administration before her boss, Atty. Gen. Eric H. Holder Jr., took office in 2009.

She said that Fast and Furious showed “weak oversight by ATF leadership” and that Holder moved swiftly to replace the agency’s management structure and instill other reforms.

“If Rep. Issa wants to continue to spend precious resources recycling old conspiracy theories for stale reports that do nothing to improve public safety, that is his prerogative,” she said. She said Holder believed “partisan exercises like Rep. Issa’s latest effort do little to address the serious issues that face our country.”

Fast and Furious was shut down after a U.S. Border Patrol agent was killed in December 2010 and two of the weapons were found nearby. Since then, Holder has reassigned top ATF managers, appointed a federal prosecutor to run the agency and made several other key changes. They include an enhanced program to assist whistle-blowers, a stronger policy on firearm transfers and additional training for federal agents.

An ATF official, who asked not to be identified because of the reforms underway and under discussion at the agency, said the changes would boost the agency’s performance and accountability, and “better serve the American people in fighting violent crime and making communities safer.”

Meanwhile, Paul Pelletier, attorney for William D. Newell, the former ATF head in Phoenix, said, “Don’t buy all this stuff Issa says,” charging that it is politically motivated to tarnish the Obama White House with the Fast and Furious scandal.

“Chairman Issa has consistently shown that he won’t let the truth get in the way of his quixotic political witch hunt,” Pelletier said. “This report continues that trend. Americans deserve better.”

But Issa and Grassley stood by their findings. “Many officials at ATF and the Department of Justice,” Issa said, “would have preferred to quietly sweep this matter under the rug.”

richard.serrano@latimes.com