Republicans on Tuesday ramped up their criticisms of the probe into the Internal Revenue Service's targeting of conservative groups after it was revealed that the Federal Bureau of Investigation does not plan to file criminal charges in the matter.

GOP leaders have long accused the Obama administration of glossing over the controversy, saying executive-branch officials whitewashed a clear attempt by the IRS to punish right-leaning groups.

Republicans are now even more incensed after a report from the Wall Street Journal said the FBI isn’t expecting to pursue criminal charges against anyone involved in the controversy.

“The administration's internal investigation - led in part by an Obama donor - has little credibility,” Brendan Buck, a spokesman for Speaker John Boehner, R-Ohio, said on Tuesday.

The apparent lack of charges comes just days after lawmakers revealed that the Justice Department official leading the investigation into the IRS' improper activity had donated money to both of President Obama's White House campaigns.

Officials found that Barbara Bosserman, of the Justice Department’s Civil Rights Division, donated $6,750 to Obama for his 2008 and 2012 presidential bids.

The DOJ said Bosserman’s appointment was proper and that it could not legally question the political donations of employees when making personnel decisions.

Conservative groups that say they were targeted, though, contend the inquiry by the administration amounts to an investigation in name only.

“What investigation?” asked Cleta Mitchell, a lawyer for many of the groups who say they received extra scrutiny from the IRS.

“You'd think they might have at least have called me or the hundreds of victims of the Tea Party scandal,” she added in an interview. “It was a very narrow, ‘if you don't look, you won't find it' strategy.”

Obama was forced to condemn the IRS after the agency revealed in 2013 that it had been unfairly targeting conservative organizations seeking tax-exempt status.

The episode fed into the narrative that the administration was inappropriately using the levers of government against ordinary Americans, charges that intensified along with the uproar over National Security Agency surveillance techniques.

However, White House officials have since dismissed the GOP’s anger over the IRS as a “phony scandal,” saying that progressive groups, not just conservative organizations, were inappropriately screened by the tax-collecting agency. They say the controversy proved the need for clearer rules over the distinction between political and charitable organizations.

White House officials Tuesday said they would not comment on an active investigation. But one senior administration official told the Washington Examiner that Republicans were engaged in a “sad, political witch hunt.”

Documents show, though, that conservative groups were targeted more heavily than liberal organizations, especially those with “Tea Party” or “Patriot” in their names.

Republicans have also made it clear they will keep their scrutiny on one of the more unpopular agencies in all of Washington.

In a midterm election year, the GOP sees a winning political argument that damages both the White House and Obama’s claim that government can be trusted to get big-ticket items done.

“These leaks come after the Justice Department, citing the confidential nature of the investigation, refused to brief Congress on its progress and congressional investigators independently discovered that a high dollar contributor to the Obama Administration failed to recuse herself,” said Rep. Darrell Issa, R-Calif., and Rep. Jim Jordan, R-Ohio, in a joint statement, about the Wall Street Journal report.

“These revelations further undermine the credibility of the Attorney General [Eric Holder] and the Justice Department under his leadership,” they added.

According to the Journal report, law enforcement officials said that investigators concluded IRS employees didn’t engage in “enemy hunting” but rather didn’t understand the rules they were trying to enforce.

When the scandal first broke, though, Obama’s surrogates used tough talk against the actions of the IRS employees.

“Those were, I think, as everyone can agree, if not criminal, they were certainly outrageous and unacceptable,” Holder said.