The addition of Peter Strzok to special counsel Robert Mueller’s probe of Russian interference in the election caused enough of a stir for CNN to note it a month ago. Strzok had overseen the FBI probe into Hillary Clinton’s infamous secret e-mail system, and had a reputation for excellence in his law-enforcement and counterintelligence work. Mueller reportedly brought in Strzok to run the investigatory side of his operation.

Less than five weeks later, ABC News reports that Strzok’s out, and no one’s quite sure why or what it means for Mueller’s investigation:

One of the FBI’s top investigators, tapped by special counsel Robert Mueller just weeks ago to help lead the probe of Russian meddling in last year’s presidential election, has left Mueller’s team, sources tell ABC News.

The recent departure of 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 Mueller is now using a federal grand jury in Washington, D.C., to collect documents and other evidence.

Why leave now? Perhaps more pressing and acute investigations required Strzok’s talents and deep experience. Or … maybe not:

It’s unclear why Strzok stepped away from Mueller’s team of nearly two dozen lawyers, investigators and administrative staff. 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.

Human resources? Granted, that’s necessary and needed work, but is it necessary to assign a top counterintelligence investigator to those tasks? Unless working HR legal issues was a childhood/law-school dream for Strzok, this looks like a demotion, and not just an issue with the probe itself. At least from appearances, Strzok must have gotten someone very angry with him. Given the nature of the probe, one would have to assume that this decision involved not just Mueller, but also recently confirmed FBI Director Christopher Wray and deputy AG Rod Rosenstein. It seems unlikely that a move of this nature didn’t get approved or perhaps even demanded at the highest levels.

So far, other than the ABC News report, those in the know have remained tight-lipped about the departure. There is also no indication that Mueller’s investigative phase of the probe has wrapped up either, and even if that was the case, it seems unlikely that Strzok would have taken an assignment in HR rather than a more high-profile position based on his earlier experience. Assuming the probe hasn’t ended, this looks like a setback for Mueller, and it may be that the investigation isn’t proceeding to his liking.