(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.
"Immediately upon learning of the allegations, the Special Counsel's Office removed Peter Strzok from the investigation," said Peter Carr, a spokesman for the special counsel's office. "Lisa Page completed her brief detail and had returned to the FBI weeks before our office was aware of the allegations."
Trump tweeted about the allegations Sunday morning.
Attorney General Jeff Sessions said in a statement Saturday night that the allegations his department's inspector general uncovered, "if proven to be true, would raise serious questions of public trust. I look forward to receiving the Inspector General's report. We will ensure that anyone who works on any investigation in the Department of Justice does so objectively and free from bias or favoritism."
"My job is to restore confidence in the Department of Justice in all aspects of our work and I intend to do so," the statement added. "As such, I have directed that the FBI Director review the information available on this and other matters and promptly make any necessary changes to his management and investigative teams consistent with the highest professional standards."
The Justice Department has also agreed for Strzok to be interviewed by the House Intelligence Committee, according to a source familiar with the matter.
The FBI said in a statement Saturday night that it has clearly defined policies and procedures on appropriate employee conduct, including communications.
"When the FBI first learned of the allegations, the employees involved were immediately reassigned, consistent with practices involving employee matters," the statement read.
The New York Times and The Washington Post earlier Saturday reported on the messages and the removal of Strzok.
The FBI declined to comment. Rumors over the cause of Strzok's removal have swirled for months.
A statement released Saturday by the Justice Department Office of the Inspector General reads, "The January 2017 statement issued by the Department of Justice Office of the Inspector General (OIG) announcing its review of allegations regarding various actions of the Department of Justice and the Federal Bureau of Investigation in advance of the 2016 election stated that the OIG review would, among other things, consider whether certain underlying investigative decisions were based on improper considerations and that we also would include issues that might arise during the course of the review. The OIG has been reviewing allegations involving communications between certain individuals, and will report its findings regarding those allegations promptly upon completion of the review of them."
Strzok's return to an FBI post in the human resources division was cast as a promotion by some at the bureau despite the fact it is widely seen to be a demotion.
It also coincided with an internal dispute as the special counsel's office pursued an aggressive strategy against Paul Manafort, the former Trump campaign manager who has since been charged with multiple counts related to his work before joining the campaign. He has pleaded not guilty to all charges.
CNN has previously reported that the IRS objected to the scope of the Manafort probe and ultimately decided to share information with Mueller's team after initially stalling. IRS agents didn't participate in a July predawn raid to search Manafort's home.
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