The ranking Democrat on the committee leading the investigation of the IRS said Sunday that the matter is effectively settled and had some unusually harsh words for committee Chairman Darrell Issa (R-Calif.).

In a letter to Issa, House oversight committee ranking member Elijah Cummings (D-Md.) expressed disappointment with the chairman's handling of the issue, suggesting he had made unsubstantiated allegations and had selectively distributed information that back up his claims, while not releasing other findings.

"Your actions over the past three years do not reflect a responsible, bipartisan approach to investigations, and the Committee's credibility has been damaged as a result," Cummings wrote.

Cummings then detailed an unreleased interview last week with an anonymous IRS manager who labeled himself a "conservative Republican." The manager essentially said it was he who started the targeting of the groups and -- contrary to GOP allegations -- said that the White House wasn't involved and that it wasn't politically motivated.

"I think this interview and these statements go a long way ... showing that the White House was not involved in this," Cummings said on CNN's "State of the Union," adding: "Based upon everything I've seen the case is solved. And if it were me, I would wrap this case up and move on, to be frank with you."

Issa, who has generally had a pleasant relationship with Cummings -- at least publicly -- responded quickly and with equal outrage.

"His extreme and reckless assertions are a signal that his true motivation is stopping needed congressional oversight and he has no genuine interest in working, on a bipartisan basis, to expose the full truth," Issa said in a statement.

Cummings's decision to release a transcript of House Oversight and Government Reform Committee investigators' interview with the IRS manager highlights Democrats' growing frustration with Issa's handling of the matter. Issa has repeatedly declined media requests to release the full transcripts of the interviews his aides conducted, instead disclosing selected excerpts.

Last week, several media outlets reported that front-line IRS employees told congressional investigators that they were simply following orders from Washington in the agency's targeting campaign against groups with names such as "tea party."

Cummings is now fighting back by releasing his excerpts of the interviews and calling for full disclosure.

Josh Hicks and Juliet Eilperin contributed to this report.