Senate Republicans have uncovered more evidence of tax misdeeds by the federal government: This time government officials violated the privacy of several political candidates and campaign donors by illegally accessing or disclosing their confidential tax records.

The Treasury Department's inspector general for tax administration also is investigating two allegations that the IRS singled out candidates for public office for a potential audit, according to Sen. Chuck Grassley, R-Iowa, the ranking member of the Senate Judiciary Committee.

So far, the Justice Department has not prosecuted any of the cases, which Grassley says is particularly troubling given the previous revelations in May that the IRS singled out conservative groups for special scrutiny.

The inspector general's findings, first reported by the Washington Times, turned up four cases since 2006 in which unidentified government officials took part in "unauthorized access or disclosure of tax records of political donors or candidates."

J. Russell George, Treasury's inspector general, described one case as particularly egregious, saying at least one government official was "willful" in accessing the person's tax records, while he said other allegations of improper access of tax records were not substantiated by the evidence.

Even though the inappropriate access most likely occurred at the IRS, the inspector general didn't name the agency or entities involved and is not releasing the names of the candidates for public office or the donors whose privacy was compromised because of taxpayer confidentiality laws.

Grassley said the violations constitute another major breach of trust by the Obama administration and is asking Justice for an explanation of its decision not to prosecute. He also accused Justice and Treasury of trying to "hide behind taxpayer confidentiality laws to avoid accountability."

"Any agency with access to tax records is required to act with neutrality and professionalism, not political bias," Grassley said. "... With the IRS on the hot seat over targeting certain political groups, it's particularly troubling to learn about 'willful unauthorized access' of tax records involving individuals who were candidates for office or political donors.

"The public needs to know whether the decision not to prosecute these violations was politically motivated and whether the individuals responsible were held accountable in any other way."

The Justice Department did not immediately return a request for comment.