(CNN) Special counsel Robert Mueller removed one of the FBI's top Russian counterintelligence experts from his team of investigators after an internal investigation found messages he sent that could be interpreted as showing political bias for Hillary Clinton and against President Donald Trump, according to US officials briefed on the matter.

Peter Strzok, who led the investigation of the Hillary Clinton email server as the No. 2 official in the FBI's counterintelligence division, left the Mueller team this past summer, multiple sources said.

Investigators from the Department of Justice Office of the Inspector General have been reviewing both the FBI's and the Justice Department's handling of the Clinton probe, which cleared the former Democratic presidential candidate of criminal wrongdoing, sources said.

The messages from Strzok to another FBI expert assigned to the Mueller team were discovered in the course of that internal review. The wording of the messages sent during the 2016 campaign appeared to be making fun of then-candidate Trump, and raised concerns that they could be seen as being pro-Clinton, the sources said.

Lisa Page, an FBI lawyer who was assigned to the Mueller investigation, received the messages. Page returned to the FBI earlier this summer.

Read More