Former Trump national security adviser Michael Flynn leaves federal court in Washington on Dec. 1. (Susan Walsh/AP)

Newly revealed text messages between two senior FBI officials could help explain why a federal judge mysteriously recused himself in December from a high-profile case involving President Trump’s former national security adviser, Michael Flynn, after Flynn lodged a guilty plea.

A judge’s recusal is unusual, but even more so just days after accepting a case and a guilty plea — Flynn admitted he lied to the FBI about his conversations with a Russian official during the transition period before Donald Trump’s inauguration.

Flynn is now cooperating with special counsel Robert S. Mueller III, who is conducting a wide-ranging investigation into Russian interference in the 2016 election and potential coordination by Trump associates with Russian officials.

The president has called the probe a “witch hunt,” and some Trump supporters who have accused Mueller and the FBI of bias in the Russia investigation have speculated whether the recusal of U.S. District Judge Rudy Contreras is related to questions about the investigators and whether they acted impartially.

The text messages show that Peter Strzok, a veteran FBI counterintelligence official who until last summer worked on the Flynn case as part of Mueller’s team, knew Contreras. The texts also appear to show him musing whether his relationship with Contreras might warrant the judge’s recusal.

The messages were exchanged July 25, 2016, between Strzok and FBI lawyer Lisa Page.

“I can’t imagine either one of you could talk about anything in detail meaningful enough to warrant recusal,” Page said to Strzok in one text, making an apparent reference to Contreras.

“Really?” Strzok replied. “ ‘Rudy, I’m in charge of espionage for the FBI.’ Any espionage [warrant request that] comes before him, what should he do? Given his friend oversees them?”

It is unclear whether any concerns about Strzok or his relationship with Contreras were behind the recusal, and it is possible that Contreras stepped down from the case in an abundance of caution, to avoid even the appearance of a conflict.

The texts, obtained by congressional investigators and shared with The Washington Post, also seem to show Strzok suggesting, perhaps in jest, meeting with Contreras at a cocktail party.

Strzok noted in one exchange that a “social setting with others would probably be better than a one on one meeting.” The message’s context suggests he was referring to Contreras, but it is unclear how seriously he was contemplating the idea. A person familiar with the matter said such a meeting between Contreras and Strzok never happened.

In January 2017, Strzok, with another FBI agent, interviewed Flynn about his conversations with Russia’s then-ambassador, Sergey Kislyak, during the transition. Flynn denied having discussed the prospect of lifting sanctions on Russia put in place by President Barack Obama. But U.S. intelligence intercepts show that the two men had in fact discussed the subject.

Strzok was reassigned to another job in the bureau in late July, and Page earlier that month voluntarily left the team; at the time no public explanation was given.

On Dec. 1, Flynn pleaded guilty to lying to the FBI. The following day, The Post and the New York Times each reported that Strzok had been removed from the Russia probe after his superiors learned that the Justice Department inspector general had obtained text messages between Strzok and Page that were politically charged.

Contreras recused himself Dec. 7.

Strzok and Contreras knew each other from living for a time in the same Fairfax, Va., neighborhood, and their wives were friends, according to two people familiar with the case.

One person said that Strzok never discussed the Flynn case with Contreras, nor did Strzok ever tell Contreras that he and Page had mentioned the judge and the possibility of his recusal in their texts. The person said Strzok did not know why Contreras recused himself from the Flynn case after hearing Flynn’s guilty plea. By that time, Strzok had been off the Russia probe for months.

Strzok, through a representative, declined to comment, as did a court spokeswoman, Mueller’s office and attorneys for Flynn.

Strzok and Page’s remarks about the judge were redacted from the initial set of messages turned over by the Justice Department to six congressional committees investigating the agencies’ handling of probes leading up and tied to the 2016 election. Congressional staffers discovered them when they were allowed by the Justice Department to review the unredacted texts. The redactions are problematic, some lawmakers said.

“The judge recused himself from the Flynn case, after he heard the case and Flynn had pleaded guilty,” said Rep. Mark Meadows (R-N.C.), a member of the House Oversight and Government Reform Committee. “The texts clearly showed there was a relationship between Strzok and the judge. And the Department of Justice made it difficult to get the information. That to me is the crux of the matter.

So far, only about 3,100 documents of more than 1 million that the inspector general has obtained related to the Justice Department and FBI investigations have been shared with congressional investigators, the lawmakers said. And most of them are highly redacted, they said.

“For many of us on Oversight, the volume of the documents and the quality of the documents that we have received to date have failed to give us a clear picture of what happened, in context, or in deed,” said Rep. Jim Jordan (R-Ohio). “We need greater cooperation between the Department of Justice and our committees to do proper oversight.”

Department officials have said they did not provide text messages that were purely personal. Moreover, they said, if a committee has specific questions about a redaction, the department will fully describe or “disclose redacted information in a closed setting.”

Judges are not required to state grounds of recusal and typically decline to do so, citing custom and the tradition of an independent judiciary.

A federal statute, however, requires that judges step back from proceedings in which their “impartiality might reasonably be questioned,” unless all parties waive such concerns after full disclosure of the issue. Judges are also required to recuse if they have a personal bias, personal knowledge of disputed facts, a financial conflict or past government service in which they worked on the matter, or if former legal colleagues or family members are involved in the case.

However, people familiar with the case said it is also possible that the judge recused when he did because it was only after the plea hearing and disclosures about Strzok that the judge learned of concerns over Strzok’s impartiality in the probe.

At one point, Page said in a text: “Standards for recusal are quite high. I just don’t think this poses an actual conflict. And he doesn’t know what you do?”

Strzok replied: “Generally he does know what I do. Not the level or scope or area. But he’s super thoughtful and rigorous about ethics and conflicts.”

The Flynn case has been reassigned to U.S. District Judge Emmet G. Sullivan.