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ABC News first reported in August that Peter Strzok, who had been tapped only weeks earlier by Mueller to help lead the probe of alleged Russian meddling in last year's presidential election, had left Mueller's team. Details about the sudden departure remained elusive at the time.
As initially reported today by The Washington Post and The New York Times, Strzok during the presidential campaign last year sent messages to a colleague that some believe could be interpreted as critical of then-candidate Donald Trump.
It's unclear if the messages continued after Trump became president and after Mueller was appointed as special counsel in May. Strzok and the person he was texting, identified by The Washington Post as FBI veteran Lisa Page, both ended up on Mueller's team.
According to the Post, Strzok and Page were involved in a romantic relationship.
"Immediately upon learning of the allegations, the Special Counsel's Office removed Peter Strzok from the investigation," Mueller spokesman Peter Carr said in a statement Saturday. "Lisa Page completed her brief [assignment] and had returned to the FBI weeks before our office was aware of the allegations."
The Justice Department's inspector general has been looking into Strzok's actions and indicated in a statement today that the review is part of broader investigations announced in January of how the FBI and Justice Department handled matters associated with the 2016 presidential election.
The inspector general's office "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," the statement said.
And on Saturday, Attorney General Jeff Sessions released a statement that he eager to "look" at inspector general's report to make sure that any Department of Justice investigation is "free from bias or favoritism."
"The allegations that the Inspector General has confirmed are part of his ongoing investigation, if proven to be true, would raise serious questions of public trust," Sessions said in the statement. "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."
He added that he asked the Inspector General to complete the report "as soon as possible" because the "American people deserve answers."
Sessions concluded in the statement that he "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.”
Meanwhile, the FBI subsequently issued its own statement on the Inspector General's inquiry on Saturday, endorsing the processes that are "designed to objectively, thoroughly and fairly determine the facts regarding potential wrongdoing."
"When the FBI first learned of the allegations, the employees involved were immediately reassigned, consistent with practices involving employee matters," the statement read. "The FBI holds all of its employees to the highest standards of integrity, independence and professionalism, as the American public rightly expects.”
Strzok has spent much of his law enforcement career working counterintelligence cases and has been unanimously praised by government officials who spoke with ABC News. He is now working for the FBI's human resources division.
He is no stranger to complex and controversial investigations.
As chief of the FBI's counterespionage section last year, he helped oversee the FBI's investigation into Hillary Clinton's use of a private email server when she was the secretary of state, and he took part in the bureau's interview of her.
Within weeks of the end of the Clinton probe, Strzok found his office facing a new challenge: Investigating Russia's alleged efforts to influence last year's presidential election, including the hacking of Democratic National Committee computers.