WASHINGTON – The Senate Ethics Committee is pushing ahead with its investigation of Sen. John Ensign, R-Nev., despite his announcement that he will resign, an unusual move that legal observers said demonstrates the panel’s resolve to at least issue a public rebuke.

With Ensign out of the Senate effective May 3, the committee will lose jurisdiction in the case and cannot formally charge him with wrongdoing in its 22-month investigation into his handling of the fallout from an affair with a political aide. Late Thursday, several hours after Ensign said he would resign, the top senators on the committee said he had made “the appropriate decision” but that their work would go on.

“The Senate Ethics Committee has worked diligently for 22 months on this matter and will complete its work in a timely fashion,” Sens. Barbara Boxer, D-Calif., chairman of the panel, and ranking member Johnny Isakson, R-Ga., said in a statement. The senators are traveling together through China as part of a 10-senator delegation.

Since June 2009, the committee has been reviewing Ensign’s affair with Cynthia Hampton, his former political treasurer, and his decision to dismiss her and her husband, Doug Hampton, who served in his legislative office until the firings in 2008. Ensign’s parents then gave a $96,000 gift to the Hamptons, and the senator also helped Doug Hampton land lobbying work with his political supporters back in Las Vegas.

Legal experts could not think of a case in recent decades in which either the Senate or House ethics committees continued their work and issued a report after a lawmaker had left office. The Ethics Committee has several options, including releasing a short statement summarizing the case or a more detailed statement that could include allegations of violations by Ensign.

Another possible route would be to send the evidence and testimony gathered in the case to the Justice Department, which has been conducting a parallel criminal inquiry. “They don’t lose the jurisdiction to make a referral,” ethics lawyer Stanley Brand said Friday.

Ethics watchdogs said Friday that Boxer and Isakson must make some public accounting for the work they have done behind closed doors. “It is critically important that the Ethics Committee follow through on their statement to complete this investigation so that the accurate, complete story can be made public. Otherwise, the mud will have been swept under the rug,” said Meredith McGehee, policy director for the Campaign Legal Center.

In his resignation statement Thursday, Ensign cited “further rounds of investigation, depositions, drawn out proceedings, or especially public hearings” as his rationale for quitting to spare his family from further public humiliation.

Ensign’s attorney, Robert Walker, declined to comment on the case, as did Ethics Committee staff members.