Michael Dreeben, the longtime U.S. Justice Department appellate lawyer who was a criminal-law expert on the special counsel's team investigating Russia's election interference, is retiring from federal government service.

Dreeben, a deputy U.S. solicitor general and career civil servant, will not return to the Justice Department's solicitor general office, where he had worked for 30 years and earned a reputation as the Justice Department's preeminent expert on criminal law.

“On behalf of the Office of the Solicitor General, I thank Michael for his many years of service to the Department. We wish him the best of luck in the next chapter of his career,” U.S. Solicitor General Noel Francisco said in a statement.

Dreeben was not reached for comment Wednesday. SCOTUSblog first reported Dreeben's plans.

Dreeben made his 100th argument at the Supreme Court in 2016, earning praise from three justices at an event marking the occasion. “I know he will tell it straight,” Justice Ruth Bader Ginsburg remarked that day at Georgetown University Law Center. Justice Elena Kagan lauded Dreeben for his “magnificent” advocacy and for helping her in 2009 and 2010 while she was serving as solicitor general.

In 1989, Dreeben argued his first case at the Supreme Court, facing off against then-advocate and now Chief Justice John Roberts Jr. in a dispute involving the False Claims Act and the double jeopardy clause of the U.S. Constitution. The court ruled against the Justice Department.

Dreeben, a 1981 graduate of Duke University School of Law, was one of two Supreme Court appellate lawyers who joined Robert Mueller III's investigative and prosecution team. The other, Elizabeth Prelogar, who is fluent in Russian, has since returned to the solicitor general's office.

“Michael Dreeben is to criminal law what Robert Mueller is to investigations,” Neal Katyal, a Hogan Lovells partner and former acting solicitor general, said in June 2017, when Dreeben joined the special counsel's team. “Literally the very best.”

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Mueller in April wrapped up his two-year investigation without making any recommendation of charges against President Donald Trump.

The investigation, captured in a 448-page report about the ties between Trump's campaign and Russian actors, and about Trump's purported effort to derail the investigation, spurred criminal cases against former advisers to Trump, and there are several cases or investigations pending in federal courts.

Other members of Mueller's team have returned to Justice Department posts, or resumed careers in private practice or at law schools. Mueller, a former Wilmer Cutler Pickering Hale and Dorr partner, has not announced his plans.