WASHINGTON — Joseph diGenova has resigned from a Justice Department investigation into the origins of the Russia-related probes that shadowed Donald Trump during his first presidential term, leaving the assignment less than five months after joining the department. DiGenova confirmed Thursday that he had stepped down but declined to explain his decision. “It was an honor and a privilege to serve the president and the department,” he said. The Justice Department did not immediately respond to requests for comment. DiGenova returned to the department in April as a counselor to then-acting Attorney General Todd Blanche. Although officials did not publicly spell out his responsibilities, he was sworn in at the U.S. Attorney’s Office for the Southern District of Florida and was photographed with attorneys involved in examining intelligence reports that helped lead the FBI to investigate Trump’s 2016 campaign and Trump himself. Blanche welcomed him at the time, writing on X, “Welcome to the fight, Joe!” The assignment drew scrutiny because diGenova, 81, had not served as a federal prosecutor for decades. He was U.S. attorney in Washington during President Ronald Reagan’s administration and later became a private legal adviser to Trump on matters arising from the Russia investigations. He also frequently defended Trump on cable television. Investigators were believed to be examining the conduct of former CIA Director John Brennan, including testimony Brennan gave Congress about intelligence assessments concerning Russian interference in the 2016 election. The investigation has not resulted in publicly announced charges against Brennan. In July, Brennan sued the Justice Department, federal officials — including diGenova — and other agencies. The lawsuit seeks to preserve records that Brennan says could be important to his defense if prosecutors charge him. U.S. District Judge Jia Cobb is scheduled to hear the case Monday in Washington. DiGenova’s sudden exit comes as the Justice Department continues to revisit the federal investigations into Russian election interference and the Trump campaign. Those inquiries, including the special counsel investigation led by Robert Mueller, examined whether Russia interfered in the 2016 election and whether Trump’s campaign coordinated with Moscow. Mueller’s report did not establish that the campaign conspired with the Russian government, while documenting extensive Russian interference efforts. The department has not said how diGenova’s resignation will affect the ongoing review or who will assume responsibility for the investigative work.