The Internal Revenue Service illegally disclosed the confidential tax records of political donors or candidates for office on four occasions since 2006, according to the Treasury Department inspector general, but the Department of Justice did not prosecute the cases.

Inspector general J. Russell George said in a letter to Republican senator Charles Grassley that his office found evidence of “willful unathorized access” in one of the four cases but, when he referred it to the Department of Justice, the DOJ declined prosecution. In the three remaining cases, the inspector general’s office determined that the access was “inadvertant.” Though TIGTA referred one of those cases to the DOJ, no prosecution occurred.

Grassley, the ranking member of the Senate Judiciary Committee, is demaning to know why attorney general Eric Holder and the DOJ did not prosecute the case. “Although this may not be indicative of widespread targeting, any instance is cause for concern,” Grassley wrote to Holder on July 12. “Even more alarming, in at least one instance TIGTA referred evidence of ‘willful unauthorized access’ to the United States Attorney’s Office, but criminal prosecution was declined.” He added, “Decisions such as these directly impact the political process and should be subject to the scrutiny of the American public.”

Despite Grassley’s inquiries, George declined to specify which political party the victims belonged to or whether he was personally aware when cases from his office were referred to the DOJ.

To date, investigations into the Internal Revenue Service have focused on its targeting of tea-party and other conservative groups. George’s disclosure marks the first indicating the agency may have targeted individuals. The Treasury Department watchdog is scheduled to testify before the House Oversight Committee on Thursday about the IRS’s targeting of tea-party groups, where he is expected to encounter fierce challenges from Democrats who have accused him of concealing evidence that the agency was targeting liberal groups, too.

TIGTA is standing by its May report, which concluded that the IRS used inappropriate criteria to screen applications for tax exemption.

UPDATE: The IRS told National Review Online in a statement that the confidential tax records in question were not disclosed by an agency employee, but were breached by a third party. “The IRS understands that the willful violation referenced in the letter was not by an IRS employee,” spokeswoman Julianna Breitbeil said. “The IRS takes its role to protect confidential taxpayer information very seriously” and “conducts annual training and certification to ensure all employees are fully aware of their responsibility to protect the confidentiality of tax return information.”