WASHINGTON — Special counsel Robert Mueller was expected to step down days after concluding his investigation in March. Yet he remains a Justice Department employee — and the department won’t say why.

Whatever role Mueller now has, keeping him on the Justice Department payroll offers one clear advantage to President Donald Trump’s administration: It makes it easier for Attorney General William Barr to block Mueller from testifying before Congress.

Democrats, who control the House, have been eager to hear from Mueller in hopes he can shine light on his investigation into interference in the 2016 presidential election, possible coordination between Russia and the Trump campaign, and potential obstruction of justice.

Barr has said repeatedly he doesn’t object to Mueller testifying. But he may defer to the wishes of Trump, who tweeted last weekend: “Bob Mueller should not testify. No redos for the Dems!”

While he’s a Justice Department employee, the department would generally handle requests for Mueller to appear before Congress, and the Justice Department could delay or block Mueller from voluntarily appearing. Congress could issue a subpoena to compel him to appear before the committee.

It isn’t clear what grounds the Justice Department would use to justify an attempt to block Mueller’s testimony.

As a private citizen, Mueller could decide whether to accept an invitation to appear or, if he declines, whether to attempt to resist any effort to subpoena him.

Rep. Jerrold Nadler, the chairman of the House Judiciary Committee, said last week the committee was “firming up the date” for Mueller’s testimony and hoping it would be May 15. If the Justice Department tries to block Mueller’s testimony, Democrats could issue a subpoena to try to compel his appearance.

Any showdown over Mueller’s testimony would add to tensions between House Democrats and the Justice Department. Barr has already defied a subpoena to provide the full, unredacted version of Mueller’s report, and Nadler has scheduled a Wednesday vote to hold the attorney general in contempt of Congress.

Mueller turned over his findings to Barr on March 22. Mueller did not find a criminal conspiracy between Russia and the Trump campaign but did not come to a definitive conclusion on whether the president obstructed justice. Two days later, Barr sent Congress a four-page letter detailing Mueller’s principal conclusion and giving his own finding that there was insufficient evidence to bring obstruction charges against Trump.