Robert Mueller has lost one of the members of his special counsel investigation into Russian tampering in the 2016 presidential election.
Experienced FBI investigator Peter Strzok has left the special counsel team and joined the bureau's human resources division, ABC News reports. It is unclear why Strzok decided to leave Mueller's probe.
The news has been rife with stories about Mueller's expanding probe into potential ties between President Donald Trump's campaign and Russia, and Strzok's departure marks the only known exit among Mueller's principal team.
Mueller's probe has grown increasingly aggressive in recent weeks. The FBI raided former Trump campaign manager Paul Manafort's home last month, and it also came out that he had impaneled a grand jury in Washington, D.C.
The grand jury has issued subpoenas related to the June 2016 meeting between Donald Trump Jr., a Russian lawyer, and others. The New York Times reported that subpoenas were also issued in relation to former national security adviser Michael Flynn.
Mueller's team of nearly two dozen lawyers, investigators, and administrative staff has been working mostly in secret since the special counsel was appointed in May. It is investigating Russia's actions as well as the possibility of collusion with the Trump campaign.
The president has criticized various aspects of the investigation, which in the past he has called a "witch hunt." He has also spoken out against Attorney General Jeff Sessions' decision to recuse himself from the investigation.