Seasoned federal prosecutor Andrew Weissmann was added to special counsel Robert Mueller’s investigative team in May after a long career spent convincing witnesses to turn on their friends, colleagues and superiors.

Weissmann’s expertise, developed during his years heading the Department of Justice’s criminal fraud section, may prove useful to Mueller’s investigation into Russian interference in the 2016 presidential election progresses.

Weissmann demonstrated his penchant for convincing people to turn on their colleagues during his involvement in the massive Enron fraud investigation and a number of New York City organized crime prosecutions. This unique ability could potentially be put to use in securing the cooperation of Trump’s inner circle, many of whom have retained their own criminal defense attorneys.

Robert Ray, who succeeded Kenneth Starr as the independent counsel investigating former President Bill Clinton, told Reuters that former National Security Advisor Michael Flynn may represent an opportunity for investigators, since the fired general has already offered to testify in exchange for immunity.

“It would seem to me the time is now to make some decisions about what you have and what leverage can be applied to get the things you don’t have,” Ray said.

Kathryn Ruemmler, who served as White House counsel under former President Barack Obama, said Weissmann is prone to taking risks to obtain witness testimony where other prosecutors may not. Ruemmler worked with Weissmann as part of a Justice Department task force that investigated Enron for corporate fraud in 2001.

She described Weissmann’s decision to call former Enron treasurer Ben Glisan to testify on a hunch that he would turn on his colleagues after previously pleading guilty and refusing to cooperate. Weissmann’s decision paid off despite skepticism among his colleagues who were concerned Glisan’s testimony might contradict the prosecution’s narrative.

“He’s not afraid to lose, and that is sometimes an unusual quality,” Ruemmler told Reuters in describing Weissmann.

Weissmann’s aggressive approach paid off again when he convinced former Enron Chief Financial Officer Andrew Fastow to testify against former Enron CEO Jeff Skelling by leveraging Fastow’s wife, also a former Enron employee convicted of tax fraud.

Prior to his work on the Enron case Weissmann helped secure the conviction of famous Mafia figure Vincent “the chin” Gigante by turning witnesses against their former boss. George Stamboulidis, who was Weissmann’s partner during the 1997 trial emphasized the importance of securing witness cooperation while prosecuting organized crime figures.

“We cut our teeth in the organized crime section,” Stamboulidis told Reuters. “And the only way you can make those cases is to get people to cooperate, even when the oath of Omerta was strong and in full play.”

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