WASHINGTON — The House Ethics Committee said Wednesday it is extending an investigation of Democratic Rep. John Conyers of Michigan, the longest-serving member of Congress, over payments to his former chief of staff.

The ethics panel said it needs more time to review a report submitted by the independent Office of Congressional Ethics, which said there is substantial reason to believe that Conyers paid his former chief of staff for work she did not perform.

Conyers, 88, is in his 27th term representing Detroit and is the top Democrat on the Judiciary Committee.

He said in April 2016 that he suspended longtime staff chief Cynthia Martin after she pleaded guilty to receiving stolen property, a misdemeanor. Martin was placed on unpaid leave while he reviewed the matter, Conyers said.

But the ethics office said Martin was paid more than $13,000 a month through August 2016. She was fired in October, although investigators said they were told as early as June 2016 that Martin no longer worked in the office.

A spokeswoman said Conyers “has worked diligently at all times to comply with” House rules and is cooperating fully with the Ethics Committee.

The investigation will be led by Chairwoman Susan Brooks, R-Ind., and Florida Rep. Ted Deutch, the panel’s senior Democrat. Brooks and Deutch said in a statement that the continuation of the inquiry does not in itself reflect any judgment of wrongdoing.

In a related development, the ethics panel also said it is continuing an inquiry of Michael Collins, chief of staff to Democratic Rep. John Lewis of Georgia.

The ethics office said there is substantial reason to believe that Collins improperly served in dual roles, in Lewis’s Washington office and as treasurer of his 2016 re-election campaign. The ethics office is an independent agency that investigates complaints against House members and staff and makes recommendation to the House Ethics Committee.

Ethics rules bar senior House staff from serving in any fiduciary role for a political organization, and specifically cite campaign treasurer as a prohibited position for such staffers. Collins, who was paid $167,500 a year as chief of staff, received nearly $55,000 over a two-year period in “campaign consulting fees,” the ethics office said.

A lawyer for Collins said he served as campaign treasurer as a volunteer and was paid for his work as a campaign strategist, a role he has long served. Collins has repaid $295, the amount he received in 2015 above a House limit for outside pay for senior staffers, attorney Robert Charrow said in a letter to the ethics panel.

Collins “takes full responsibility for the overpayment, which is unlikely to recur because the campaign committee is establishing safeguards to more closely track payments” to Collins, the lawyer wrote.