The people briefed on the case said the transfer followed the discovery of text messages in which Mr. Strzok and a colleague reacted to news events, like presidential debates, in ways that could appear anti-Trump. A review of the texts is now underway to determine whether the two agents allowed political bias to influence their work, but regardless of what that investigation concludes, the story is sure to provide ammunition for those, like President Trump and his defenders, who have accused the Mueller investigation, FBI, and overall intelligence community of being driven by partisanship.
A friend of Page said there was no professional misconduct at issue in the matter, and she said Page left the Mueller team in July before any of the issues came to light.
Strzok's attorney refused to comment on the publication.
Defenders of Page and Strzok insisted the issue is "overblown" and that there was no misconduct between the two. A Justice Department spokesperson stated: "We are aware of the allegation and are taking any and all appropriate steps".
Peter Strzok, a veteran FBI investigator, was shuffled off to the FBI's human resources department by Mueller after the Justice Department's inspector general opened an investigation into the texts, The New York Times reported on Saturday. Word of the texts could give new fuel to those demands.
Mr Trump wrote that after years under fired FBI director James Comey, "with the phony and dishonest Clinton investigation (and more)", the agency's reputation "is in Tatters - worst in History!" But the issue was resurrected in late October, when Comey informed congressional leaders that the FBI had discovered new Clinton emails on a laptop belonging to former congressman Anthony Weiner, then the subject of a separate FBI investigation. Mr. Trump seriously entertained the idea but ultimately backed down.
The president said earlier on Sunday that he had never asked Mr Comey to stop investigating ex-national security adviser Michael Flynn.
Mr Flynn pleaded guilty on Friday to lying to the FBI and agreed to co-operate with Robert Mueller's investigation.
Strzok was a major player in the Clinton email and Trump-Russia investigations.
A former senior Trump administration official said Strzok was even-handed in all of his dealing with the Trump White House.