The F.B.I.’s acting director abruptly retired on Monday as the bureau faces a potentially tumultuous leadership change under the Trump administration.

The move left unclear who would take over once the departure of the acting director, Paul Abbate, took effect at noon. His exit is unusual given that the acting director typically remains in place until a new director is confirmed by the Senate to ensure stability at the highest ranks of the bureau.

Mr. Abbate took command of the bureau on Sunday after Christopher A. Wray, the previous director, announced last month that he would step down before the inauguration. President Trump appointed Mr. Wray in 2017 but publicly attacked him and the F.B.I., which repeatedly investigated him.

In a rare move, Mr. Wray extended the service of Mr. Abbate, who is 57, the mandatory retirement age of certain F.B.I. agents.

In an email to senior F.B.I. officials, Mr. Abbate wrote: “When the director asked me to stay on past my mandatory date for a brief time, I did so to help ensure continuity and the best transition for the F.B.I. Now, with new leadership inbound, after nearly four years in the deputy role, I am departing the F.B.I.”

He added, “I have complete confidence in you and in your ability as a team to continue to carry out our mission of protecting the American people and upholding the Constitution.”

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