The recent departure of the FBI veteran, Peter Strzok, is the first known hitch in a secretive probe that, by all public accounts, is charging full steam ahead. Just last week, news surfaced that Mueller's team had executed a search warrant at the Virginia home of Donald Trump's former campaign manager Paul Manafort. And the week before that, ABC News confirmed that Mueller is now using a federal grand jury in Washington, D.C., to collect documents and other evidence.
It's unclear why Strzok stepped away from Mueller's team of nearly two dozen lawyers, investigators and administrative staffers. Strzok, who has spent much of his law enforcement career working counterintelligence cases and has been unanimously praised by government officials who spoke with ABC News, is now working for the FBI's human resources division.
He is no stranger to complex and controversial investigations.
As chief of the FBI's counterespionage section last year, he helped oversee the FBI's investigation into Hillary Clinton's use of a private email server when she was the secretary of state, and he took part in the bureau's interview of her.
Within weeks of the end of the Clinton probe, Strzok found his office facing a new challenge: investigating Russia's alleged efforts to influence last year's presidential election, including hacking of Democratic National Committee computers.
A spokesman for the special counsel's office declined to comment.