Andrew Weissmann, the architect of the case against former Trump campaign chairman Paul Manafort, will study and teach at New York University and work on a variety of public service projects, including his longstanding interest in preventing wrongful convictions by shoring up forensic science standards used in courts, the sources added.
The departure is the strongest sign yet that Mueller and his team have all but concluded their work.
Weissmann has borne the brunt of attacks from critics such as Rush Limbaugh and conservative legal interest groups.
They cited his attendance at Hillary Clinton's election night party in 2016 and a positive email he wrote to former Acting Attorney General Sally Yates after she refused to defend the Trump administration's first Muslim travel ban.
Former Trump adviser Steve Bannon also issued a warning about Weissmann and other senior members of the special counsel team when they were named in 2017.
Trump and his aides would be facing off against a group of "killers," Bannon reportedly said. Author Michael Wolff wrote that Bannon told him that Weissmann was like "the LeBron James of money laundering investigations."
Former Enron prosecutor Kathryn Ruemmler said there's a reason for the attacks on Weissmann.
"Andrew is attacked because he is feared; those under investigation know just how effective he is," Ruemmler said. "He has not only peerless technical skills, but the fearlessness necessary for pursuing high profile, complex cases and a passionate commitment to seeing justice is done."
Leslie Caldwell worked alongside Weissmann in Brooklyn and at the Justice Department in Washington. She said he has a reputation for getting results.
"Throughout his career, Andrew has had unparalleled success in building case after case against the most sophisticated criminals in the world," Caldwell said. "He took on New York's most feared organized crime families, unraveled the incredibly ornate frauds at Enron, and has tracked international criminals, exposing their carefully concealed financial dealings in many dark corners of the world."
His leaving will follow the departure of the senior-most FBI agent working on the Mueller probe, who has taken his own next step. Special Agent in Charge David Archey started a new job on March 4 as head of the FBI's office in Richmond, Va.
Earlier this month, another special counsel prosecutor, Brandon Van Grack, moved on to lead a Justice Department effort to enforce compliance with the Foreign Agents Registration Act, a law that has become the subject of intense interest following charges against Manafort, his right-hand-man Richard Gates, and former Trump national security adviser Michael Flynn.
And WilmerHale, the law firm that Mueller and several other prosecutors left to help create the special counsel team, is preparing for the return of some of its onetime law partners, three lawyers have told NPR in recent weeks.