A lawyer for Mr. Strzok did not return several messages seeking comment. A spokeswoman for the Justice Department said that “we are aware of the allegation and are taking any and all appropriate steps.” ABC News reported in August that Mr. Strzok had left the investigation, but the reason for the move was unclear at the time.
Mr. Strzok’s reassignment shows that Mr. Mueller moved swiftly in the face of what could be perceived as bias by one of his agents amid a politically charged inquiry into Mr. Trump’s campaign and administration. But the existence of the text messages is likely to fuel claims by Mr. Trump that he is the target of a witch hunt.
The discovery of the text messages came at a crucial moment in Mr. Mueller’s investigation. At the time, Mr. Mueller was ramping up his inquiry into Mr. Trump’s former advisers, while also coming under criticism for putting many donors to Democratic candidates on his team. Some conservatives encouraged Mr. Trump to fire Mr. Mueller, saying the investigation was tainted. Mr. Trump seriously entertained the idea but ultimately backed down.
It is not clear what Mr. Strzok said in his text messages. F.B.I. regulations allow agents to express opinions “as an individual privately and publicly on political subjects and candidates.”
Current and former law enforcement officials who worked with Mr. Strzok said they had never seen any evidence that he allowed his political views to influence the investigations he led. They said that Mr. Strzok, a former Army officer, was deeply trusted by James B. Comey, the former F.B.I. director, and that he would probably have become one of the bureau’s top officials had the questions not been raised about his text messages.
The inquiry into Mr. Strzok is being conducted by the Justice Department’s inspector general, Michael E. Horowitz, who is leading a broad examination of how the F.B.I. handled the Clinton email investigation. It is not clear whether Mr. Horowitz will make the text messages public as part of his report in the inquiry. Mr. Horowitz, who announced the beginning of his investigation in January, declined to characterize his findings but said that he hoped to have a copy of his report released by March or April.