WASHINGTON -- House Oversight Committee Chairman Darrell Issa (R-Calif.) contradicted House Speaker John Boehner (R-Ohio) on Sunday, saying that Congress has found no evidence that President Barack Obama or anyone at the White House had participated in a cover-up involving the Fast and Furious scandal.

On Thursday, Boehner said that the Obama administration's decision to invoke executive privilege to prevent the release of some documents about the program Issa had requested was "an admission that White House officials were involved in the decision that misled the Congress and covered up the truth."

During an appearance on "Fox News Sunday," host Chris Wallace asked Issa whether he had "any evidence" that the White House had been involved with the matter.

"No, we don't," Issa said.

The Justice Department's Bureau of Alcohol, Tobacco, Firearms and Explosives apparently allowed some gun sales in Arizona to known Mexican criminals. Two of the guns involved were found at the scene of a firefight in which U.S. Border Patrol Agent Brian Terry was killed.

In February 2011, DOJ sent a letter to Congress denying such sales, but retracted that claim in December. It is not clear if either Attorney General Eric Holder or President Barack Obama were aware of the program, and Issa acknowledged Sunday that he has not found evidence Holder was involved.

"I did not say that Eric Holder always knew," Issa said.

But the investigation has devolved into a bizarre, complex conspiracy theory, which Issa and other congressional Republicans have suggested the Obama administration hatched in order to impose new gun control regulations. By inciting gun violence in Mexico, the theory goes, the administration hoped to spark a controversy that would build public support for gun control.

In April, Issa presented the case at a National Rifle Association conference.

"Could it be that what they really were thinking of was in fact to use this walking of guns in order to promote an assault weapons ban? Many think so," Issa said at the time. "And they haven't come up with an explanation that would cause any of us not to agree."

He defended the conspiracy theory Sunday on ABC's "This Week," saying that emails about the aftermath of Fast and Furious indicate that people reviewing the results of the program had the power to inform new regulatory actions. That does not, however, mean that the program was developed for the purpose of imposing gun control rules. President George W. Bush's administration opposed gun control, and it used similar programs to track criminal groups.

"We have emails from people who are involved in this who are talking about using what they're finding to support an assault weapons ban," Issa said. "So chicken or egg, which came first, we'll probably never know."

Issa also acknowledged on the show that previous gun-walking cases occurred during the Bush administration, but said the practice went further under Obama.

"It started under Bush, was closed up under Bush, reopened under President Obama, and in fact got to be on steroids," he said.

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  • From left, House Minority Whip Steny Hoyer of Md., Rep. Maxine Waters, D-Calif., Rep. John Lewis, D-Ga., Rep, Charles Rangel, D-N.Y., and House Minority Leader Nancy Pelosi of Calif., right, and many House Democrats walk out of the Capitol during the vote to hold Attorney General Eric Holder in contempt, Thursday, June 28, 2012, on Capitol Hill in Washington. (AP Photo/J. Scott Applewhite)

  • House Oversight and Government Reform Committee Chairman Rep. Darrell Issa, R-Calif., left, and the committee's ranking Democrat Rep. Elijah Cummings, D-Md., go to the House Rules Committee, on Capitol Hill in Washington, Wednesday, June 27, 2012, to argue procedures as the House of Representatives prepares to vote on whether Attorney General Eric Holder is in contempt of Congress because he has refused to give the Oversight Committee all the documents it wants related to Operation Fast and Furious, the flawed gun-smuggling probe involving Mexican drug cartels. (AP Photo/J. Scott Applewhite)

  • Rep. Carolyn Maloney, D-N.Y., left, stretches out her hand to get the attention of Rep. Darrell Issa, R-Calif., chairman of the House Oversight and Government Reform Committee, as the panel considers a vote to hold Attorney General Eric Holder in contempt of Congress, on Capitol Hill in Washington, Wednesday, June 20, 2012. Rep. Elijah Cummings, D-Md., the ranking member, sits between them. (AP Photo/J. Scott Applewhite)

  • WASHINGTON, DC - JUNE 20: House Oversight and Government Reform Chairman Darrell Issa (R-CA) (C) talks with raking member U.S. Rep. Elijah Cummings (D-MD) (L) during a mark up hearing on Capitol Hill June 20, 2012 in Washington, DC. Issa and the committee Republicans called the hearing to vote on holding U.S. Attorney General Eric Holder in contempt of Congress for refusing to hand over documents the GOP says are key to their investigation into the failed Fast and Furious operation. Before the start of the hearing, the White House asserted the documents are protected by executive privilidge. (Photo by Chip Somodevilla/Getty Images)

  • Rep. Darrell Issa, R-Calif., center, chairman of the House Oversight and Government Reform Committee, confers with an aide as the panel considers a vote to hold Attorney General Eric Holder in contempt of Congress, on Capitol Hill in Washington, Wednesday, June 20, 2012. Rep. Elijah Cummings, D-Md., the ranking member, second from left, speaks with Rep. Carolyn Maloney, D-N.Y., who authored an amendment in support of Holder. In a showdown with President Barack Obama's administration, House Republicans had pressed for more Justice Department documents on the flawed gun-smuggling probe known as Operation Fast and Furious that resulted in hundreds of guns illicitly purchased in Arizona gun shops winding up in the hands of Mexican drug cartels. (AP Photo/J. Scott Applewhite)

  • Rep. Jason Chaffetz, R-Utah, center, debates Rep. Stephen Lynch, D-Mass., as the House Oversight and Government Reform Committee considers a vote to hold Attorney General Eric Holder in contempt of Congress, on Capitol Hill in Washington, Wednesday, June 20, 2012. (AP Photo/J. Scott Applewhite)

  • The House Oversight and Government Reform Committee, led by Chairman Darrell Issa, R-Calif., right, considers whether to hold Attorney General Eric Holder in contempt of Congress, on Capitol Hill in Washington, Wednesday, June 20, 2012. Rep. Elijah Cummings, D-Md., the ranking member, speaks at left. In a showdown with President Barack Obama's administration, House Republicans are pressing for more Justice Department documents on the flawed gun-smuggling probe known as Operation Fast and Furious that resulted in hundreds of guns illicitly purchased in Arizona gun shops winding up in the hands of Mexican drug cartels. (AP Photo/J. Scott Applewhite)

  • WASHINGTON, DC - JUNE 20: House Oversight and Government Reform raking member U.S. Rep. Elijah Cummings (D-MD) (R) hears from U.S. Rep. Dennis Kucinich (D-OH) during a mark up hearing with June 20, 2012 in Washington, DC. Chairman Darrell Issa (R-CA) and the committee Republicans called the hearing to vote on holding U.S. Attorney General Eric Holder in contempt of Congress for refusing to hand over documents the GOP says are key to their investigation into the failed Fast and Furious operation. Before the start of the hearing, the White House asserted the documents are protected by executive privilidge. (Photo by Chip Somodevilla/Getty Images)

  • Attorney General Eric holder speaks to reporters following his meeting on Capitol Hill in Washington, Tuesday, June 19, 2012. Holder wants a House panel to drop plans to try to hold him in contempt of Congress, and the panel's chairman wants more Justice Department documents regarding Operation Fast and Furious, a flawed gun-smuggling probe in Arizona. Holder and Rep. Issa met in an effort to resolve their dispute over the investigation of Fast and Furious by the House Oversight and Government Reform Committee that Issa chairs.(AP Photo/Susan Walsh)

  • Rep. Darrell Issa, R-Calif., speaks to reporters following his meeting with Attorney General Eric Holder on Capitol Hill in Washington, Tuesday, June 19, 2012. Holder wants a House panel to drop plans to try to hold him in contempt of Congress, and the panel's chairman wants more Justice Department documents regarding Operation Fast and Furious, a flawed gun-smuggling probe in Arizona. Holder and Issa met in an effort to resolve their dispute over the investigation of Fast and Furious by the House Oversight and Government Reform Committee that Issa chairs. (AP Photo/Susan Walsh)

  • Attorney General Eric Holder speaks to reporters following his meeting on Capitol Hill in Washington, Tuesday, June 19, 2012. Holder wants a House panel to drop plans to try to hold him in contempt of Congress, and the panel's chairman wants more Justice Department documents regarding Operation Fast and Furious, a flawed gun-smuggling probe in Arizona. Holder and Rep. Darrell Issa, a California Republican, met in an effort to resolve their dispute over the investigation of Fast and Furious by the House Oversight and Government Reform Committee that Issa chairs. (AP Photo/Susan Walsh)

  • Attorney General Eric Holder walks through Statuary Hall to speak to reporters following his meeting on Capitol Hill in Washington, Tuesday, June 19, 2012. (AP Photo/Susan Walsh)

  • FILE -In this Dec. 8, 2011 file photo, House Judiciary Committee member Rep. Darrell Issa, R-Calif., left, shakes hands with Attorney General Eric Holder on Capitol Hill in Washington. Holder is proposing to meet with Issa by Monday to settle a dispute over Justice Department documents the congressman is demanding on a flawed gun-smuggling probe. (AP Photo/Susan Walsh, File)