WASHINGTON — It’s been nearly six years since the Senate Ethics Committee conducted a major investigation of a sitting senator. Next year, the panel could be working nonstop, deciding the fate of up to three lawmakers, including two facing allegations of inappropriate sexual behavior.

The typically secretive committee of three Republicans and three Democrats said late Thursday it plans to resume its preliminary inquiry into alleged misconduct by Sen. Bob Menendez, D-N.J., whose federal bribery trial ended in a mistrial. The panel had begun an investigation in 2012, but deferred to the Justice Department for its probe.

Delving into the onslaught of allegations of sexual misconduct by powerful figures, the ethics panel is expected to investigate Minnesota Sen. Al Franken after a woman accused him of forcibly kissing her and groping her during a 2006 USO tour. Franken, a Democrat, has said he welcomes the probe.

The Senate is likely to enter uncharted territory on the case of Alabama’s Roy Moore, a Republican who faces multiple complaints from women who said he pursued them when they were teens and he was in his 30s. If Moore wins the Dec. 12 special election, the top Senate Republican says he would immediately face a formal ethics complaint.

“He would be sworn in and be asked to testify under oath and it would be a rather unusual beginning, probably an unprecedented beginning,” Majority Leader Mitch McConnell, R-Ky., said this week at a Wall Street Journal event.

The flurry of activity is unusual for the panel, which until Thursday had not issued a press release since hiring a new staff director in 2014. The panel’s last major investigation focused on John Ensign, a Nevada Republican who resigned in 2011 after revelations that he had an affair with the wife of a top staffer.

Disclosure of the affair and Ensign’s actions to keep it quiet, including accusations that he helped the staffer find work as a lobbyist, resulted in investigations by the FBI, Federal Election Commission and the Senate. Ensign resigned as the two-year ethics investigation intensified.

The members of the committee have changed since then. The panel is chaired by Sen. Johnny Isakson, R-Ga., and Sen. Chris Coons, D-Del., is vice chairman. Other members are Republican Sens. Pat Roberts of Kansas and Jim Risch of Idaho, along with Democrats Brian Schatz of Hawaii and Jeanne Shaheen of New Hampshire.

Robert L. Walker, a former chief counsel for the ethics panel, said senators who serve on the committee typically are respected by their peers. McConnell served on it, overseeing the investigation of Oregon Sen. Bob Packwood.

“I don’t think it’s an assignment anyone relishes. No one relishes being in a position to pass judgment on others, especially one of their peers,” Walker said. “But they understand the importance and ultimate seriousness of this assignment.”

Among the committee’s responsibilities are dealing with Senate offices on gifts, travel, compliance with rules and potential conflicts of interests. Major investigations such as the Ensign or Packwood probes can take years to complete.

In 2008, the ethics panel admonished then-Idaho Sen. Larry Craig, saying he acted improperly in connection with a men’s room sex sting and brought discredit on the Senate.

In a letter to the Republican senator, the ethics panel said Craig’s attempt to withdraw his guilty plea after his 2007 arrest at a Minneapolis airport was an effort to evade legal consequences of his own actions. Craig initially announced he would resign his Senate seat, then reneged and served out his term.

More than a decade earlier, the ethics panel found itself in the midst of another sex scandal involving Packwood. In a report delivered by then-ethics chairman McConnell, the committee described Packwood’s “physical coercion” of women and “a habitual pattern of aggressive, blatantly sexual advances, mostly directed at members of his own staff.”

The veteran Republican resigned in 1995 under threat of expulsion after a nearly three-year investigation of sexual harassment claims.

The ethics committee typically gets dozens of complaints each year alleging violations of Senate rules, but the vast majority don’t amount to a violation of Senate rules or there is too little evidence to take action.

The committee said in an annual report that it received 63 complaints. Of that amount, 43 were dismissed for lack of jurisdiction, 14 were dismissed because they didn’t provide sufficient facts to follow and three were dismissed as minor or technical. Also, the staff undertook a preliminary investigation in three cases that originated that year and found no major violation.

If the committee finds a violation occurred, it may take a series of actions, including issuing a public or private letter of admonition or recommending disciplinary action by the full Senate, up to and including expulsion on a two-thirds vote. Since 1789, the Senate has expelled just 15 members, including 14 who were charged with support of the Confederacy during the Civil War.