The House Ethics Committee announced Wednesday that it would hire prominent D.C. attorney Billy Martin to serve as an outside counsel in its troubled investigation of Rep. Maxine Waters (D-Calif.).

The bipartisan committee’s unanimous decision — revealed in a joint statement from Chairman Jo Bonner (R-Ala.) and ranking member Linda Sanchez (D-Calif.) — comes in the wake of Waters’ attorney demanding that the entire case be dismissed because of irregularities in how the committee has handled the probe.

“The Committee’s decision reflects the high priority of this unique matter and the need to resolve it with the utmost care, diligence and integrity,” Bonner and Sanchez said, adding: “Serious allegations have been made about the Committee’s own conduct in this matter by Representative Waters and others. The Committee does not take these allegations lightly.”

In a press release, Waters said the Ethics panel’s step was “a recognition by the committee, that its investigation of me was misguided, flawed and could go no further. I am confident that the counsel’s review of the committee’s misconduct will conclude that my rights were violated and further investigation of me is not warranted.”

Waters has been under investigation by the committee since 2009 for her alleged role in arranging federal help for OneUnited, a minority-owned bank in which her husband had a financial interest. The panel was not able to complete its investigation in the 111th Congress and so was forced to renew its probe this year.

The committee has been racked by partisan disputes during the investigation, particularly on the staff level, with allegations flying that multiple former committee aides improperly shared information about the probe with lawmakers.

The panel has hired outside counsels in past probes, and Martin — a partner at Dorsey & Whitney LLP — will be tasked with reviewing all of the allegations of impropriety within the committee and then reporting his conclusions to panel members, who will decide whether to continue the Waters probe.

The decision to hire Martin was announced the same day that a coalition of government-watchdog groups urged the panel to hand the investigation over to an independent counsel.

A copy of Martin’s contract with the Ethics Committee shows that the panel is authorized to pay the outside lawyer up to $500,000 for his work. The contract lasts through the start of 2012, though it can be terminated at any time by order of Bonner, the committee chairman.