One of the investigators recruited to serve on Robert Mueller's special counsel has left the investigation.

Longtime FBI investigator Peter Strzok has stepped away from the investigation that seemed to be hitting a new stride in recent weeks, according to ABC News. It's not clear what motivated the departure.

The special counsel investigation made headlines in early August when it was learned that Mueller had impaneled a grand jury in the probe. Later it was learned that the FBI had raided the Virginia home of former Trump campaign manager Paul Manafort.

Strzok's departure, then, becomes one of the only hiccups for the team that's charged with looking into Russian meddling in the 2016 elections, and whether any of that meddling might have been in coordination with the Trump team.

Other Trump associates, like former communications consultant Michael Caputo who testified to the House Intelligence Committee, have said they have had no contact from the Mueller investigation.

Strzok has previously led the FBI's counter-espionage section, and also worked on the Hillary Clinton email investigation.