A top FBI official who helped oversee two politically sensitive investigations related to the 2016 presidential campaign is retiring from government service.

Bill Priestap, who currently serves as assistant director of the Federal Bureau of Investigation’s counterintelligence division, will leave his post by the end of the year. Mr. Priestap, a 20-year veteran of the bureau, worked on organized crime and drug cases in Chicago before rising through the national security ranks of the agency after the terrorist attacks of Sept. 11, 2001.

Mr. Priestap’s retirement is unrelated to the controversies over the handling of the 2016 investigations, according to a person familiar with the matter. He “became eligible to retire and has chosen to do so after 20 years of service,” the FBI said in a statement.

The federal government allows some employees, including FBI agents, to retire with full benefits if they are 50 or older and have at least two decades of service.

During the 2016 campaign, Mr. Priestap was one of several officials at the center of two politically volatile probes: the investigation into Hillary Clinton’s handling of classified information, and a counterintelligence inquiry into whether associates of then-candidate Donald Trump colluded with the Russian government.

After Mr. Priestap’s departure, none of the high-ranking bureau officials involved in the two investigations will remain with the bureau. FBI director James Comeywas fired by President Trump last year, and Deputy Director Andrew McCabewas later dismissed by then-Attorney General Jeff Sessions over his contacts with the media, days before he was eligible to retire with benefits.

Peter Strzok, the chief of the counterespionage section, left the FBI this year after it emerged that he had sent disparaging text messages about Mr. Trump.

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