WASHINGTON The relationship between James Comey and Hillary Clinton was never going to be tension-free, not when Comey’s FBI had conducted an election-year criminal investigation into the Democratic presidential candidate’s email practices.

But Comey’s sudden announcement to Congress that FBI agents would review new emails that may be connected to that dormant investigation revives questions about how Clinton, if elected, would coexist with the independent-minded FBI director. Comey has shown a willingness to break with the White House and has been critical of her handling of sensitive information as secretary of state.

The FBI director is appointed to 10-year terms, to avoid any appearance of political influence. Comey took over in September 2013, meaning he still would be on the job if Clinton is sworn into office in January. That could raise the prospect of an unmistakably fragile dynamic, but it probably would not be any easier if Republican Donald Trump won, given his criticism of the FBI after Comey’s recommendation in July against prosecuting Clinton in the email matter.

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“There needs to be a mutual trust between a president and an FBI director given the importance of that post,” said Jamie Gorelick, deputy attorney general during former President Bill Clinton’s administration.

Ron Hosko, a former assistant FBI director under Comey, said he envisioned a “very, very difficult relationship,” but Comey’s ability to compartmentalize his duties would enable him to keep doing his job.

“Could Jim Comey go over to the White House and brief on terrorism intelligence or a terrorism strike, and what the bureau is doing about it or has done about it, and keep that in a separate box? Yes,” Hosko said.

Clinton and her campaign have aggressively challenged the FBI to release more information about the emails, presumably because they believe a fuller portrait would prove exculpatory. Clinton on Saturday called it “pretty strange to put something like that out with such little information right before an election.” Her campaign chairman, John Podesta, called the letter an “unprecedented step” that cried out for more clarity.

Comey acknowledged in a memo Friday that his letter created the risk of being misunderstood so close to the Nov. 8 election. But he said he felt obligated to alert Congress to the new emails, which surfaced during an unrelated criminal sexting investigation involving former Rep. Anthony Weiner, D-N.Y., after having previously told lawmakers that the email investigation had been concluded. Keeping the emails secret until after the election carried its own political risks.

Late Sunday, a law enforcement official confirmed the FBI had obtained a warrant to begin reviewing the emails, which he said would be done expeditiously. The official had knowledge of the investigation, but was not authorized to speak of it publicly and requested anonymity to do so. It’s not clear what the emails are about, whether they contain classified information or how they’re connected to the Clinton investigation, which examined the mishandling of classified information on her private server. Comey could not guarantee that the review would be done by Election Day.

Conflict between a president and an FBI director is not without precedent.

Bill Clinton had a notoriously tense relationship with his FBI director, Louis Freeh. Clinton devoted multiple paragraphs in his 2004 memoir to castigating Freeh for various decisions. Freeh, for his part, resigned before his 10 years were up, and has said he wore Clinton’s criticism as a “badge of honor.”

When Comey was nominated for the job in 2013, President Barack Obama praised him for his “fierce independence and deep integrity.” A former Republican who served as deputy attorney general under President George W. Bush, Comey has said he’s no longer registered with a political party.

He famously split with the White House in 2004 over the authorization of a domestic surveillance program, leading to a remarkable confrontation in the hospital room of then-Attorney General John Ashcroft. Comey called the event “probably the most difficult night of my professional life.”

As FBI director, he has floated the possibility that police concerns over being recorded on viral videos were causing officers to pull back and contributing to an uptick in homicides, a viewpoint the White House refused to endorse.

And in the Clinton matter, Comey testified before Congress for more than four hours after announcing his recommendation against prosecution at an FBI headquarters news conference, where he rebuked her and her aides as “extremely careless” in the handling of classified information.

On Friday, Comey sent Congress a vaguely worded, three-paragraph letter that went against the counsel of Justice Department leaders, who believed the action was inconsistent with longstanding department policy meant to avoid the appearance of prosecutorial interference in elections.

Now, the same independence that’s earned Comey bipartisan praise has given way to intense criticism over the timing of his action from Democrats and some Republicans.

Hosko said the Clinton team may be tempted to seek Comey’s departure out of concern that the FBI will be “on our back” in the future. But he said that would be a foolish decision.

“If I replace him, replace him with who?” he said. “Someone who’s going to tank it? How’s that going to play?”