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But Mr. Carter said his agency did not engage in wrongdoing.

“Upon reviewing the access with the U.S. Treasury Department in December 2012, my division again found state access of the records of the taxpayer was part of a typical review and was not improper,” Mr. Carter said. “Further, the state Division of Revenue has received no notification from the U.S. Treasury that access of the taxpayer’s records by a state employee was improper.”

Mr. Carter said the division “routinely compares state and federal tax records for discrepancies and potential investigation based on information provided on tax forms, financial information from other state government agencies, from tips or complaints, or from media coverage or other public information highlighting tax issues.”

Ms. O'Donnell said the explanation from Delaware’s tax office “simply raises more questions.”

“It all points to a need to continue the investigation to get to the bottom of this,” she said.

Mr. George also told Mr. Grassley that his office is investigating two other allegations that political candidates were targeted for IRS audits.

The Iowa Republican has asked the Justice Department to respond to him by Friday on the matter. A spokesman for the Justice Department said in an email that they have received Mr. Grassley’s request and are reviewing it, declining further comment.

Oversight Committee ranking member Elijah Cummings, Maryland Democrat, was also irked that Mr. George apparently intervened and blocked the release of further information about possible political targeting to the oversight committee. Mr. George said he did so because he received conflicting information about whether the information could be released and that he wanted to err on the side of caution in not releasing sensitive taxpayer records.

Though there is still no indication the tax-scrutiny fiasco went as far up as the White House, the bipartisan interest indicates that it is unlikely to fade away anytime soon.

“[N]obody’s trying to attack you or anybody else — just trying to get to the facts,” Mr. Cummings told Mr. George at the oversight committee hearing last week. “This is our watch. This is it. I’ve said over and over again, we don’t know how long we are going to be here.”