The Justice Department is facing mounting criticism after officials said they've turned up no evidence that would warrant criminal charges in the IRS targeting scandal, with conservatives now calling the investigation a "sham."

Officials said earlier this week that, while the FBI investigation into IRS targeting of Tea Party and other groups is still ongoing, criminal charges are not expected at this point.

Jenny Beth Martin, the co-founder of the Tea Party Patriots, called the news "absolutely outrageous."

"It only leads us to the point where we can make guesses about their motives," she said. "Why are they protecting the IRS? Why aren't they getting to the bottom of it? If they don't truly do a real investigation of this there are always going to be doubts and it is not good for the trust between the governors and the governed."

Martin said she believes the administration is attempting to sweep the scandal under the rug, and it appears there is a cover-up.

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"They wanted to silence us and they have done what they can to silence us," she said. "We're not going away, but they've done everything they can to make it difficult for us if not impossible at times to exercise our First Amendment rights."

The complaints come after it was revealed that the woman leading the FBI probe is a President Obama donor. Republicans, to no avail, have urged the Justice Department to take her off the case.

The developments have fueled claims that there's been a lack of accountability in the administration over the scandal. The toughest action President Obama took was last May, when he announced Acting IRS Commissioner Steven Miller resigned at the administration's request following revelations that the agency singled out conservative groups for additional scrutiny as they sought tax-exempt status. However, a source told Fox News at the time that Miller was set to resign the following month anyway. Other lower-level officials have faced discipline or left the administration, though the circumstances were unclear.

Jay Sekulow, chief counsel with the American Center for Law and Justice which is representing Tea Party groups, called the investigation a "sham."

"To reach a conclusion that no criminal charges will be filed against those responsible for this illegal targeting scheme without interviewing any of our clients -- the real victims in this ordeal -- is absolutely absurd," he said in a statement.

Sen. Orrin Hatch, R-Utah, questioned how sources could be discussing the likelihood of criminal charges in the first place when the probe isn't even finished.

"These are serious matters - but to make this decision before the investigation is completed seems entirely out of order and inappropriate," he said.

The Wall Street Journal first reported that criminal charges were unlikely, unless investigators discover evidence that dramatically changes their viewpoint.

Sources separately told Fox News that the investigation is still ongoing, but that they've turned up no evidence yet to support criminal charges.