Stephen K. Bannon, President Trump’s former chief strategist and former executive chairman of Breitbart News, was subpoenaed last week by special counsel Robert Mueller to testify before a grand jury in its Russia probe, according to the New York Times.

The move is reportedly the first time Mueller has used a grand jury subpoena to seek information from a member of Trump’s “inner circle.”

The subpoena could be a negotiating tactic, the Times wrote. Mueller is “likely” to allow Bannon to forgo the grand jury appearance if he agrees, instead, to be questioned by investigators in a less formal setting, according to a person directly familiar with the matter, who told the Times about the subpoena.

The subpoena also indicates that Bannon is not a target of the special counsel because Justice Department rules allow prosecutors to subpoena targets only in rare circumstances.

Peter Carr, spokesman for the special counsel, declined to comment on the ongoing investigation.

It is not clear why Mueller issued a subpoena instead of a request for an interview as with a dozen other administration officials who have been interviewed. One theory is that it could give Bannon some cover for testifying to the special counsel because he had no choice.

The subpoena comes after Bannon was quoted in a new book, Fire and Fury: Inside the Trump White House, calling Donald Trump Jr.’s meeting with Russians in June 2016 at Trump Tower “treasonous.” Bannon also allegedly said there was “zero” chance that Trump Jr. did not walk the Russians to meet his father.

Since the book was published, Bannon has clarified those comments, saying they were directed to then-campaign manager Paul Manafort, not Trump Jr.

The meeting predated Bannon’s joining the Trump campaign as executive chair, but Bannon’s secondhand knowledge can be contrasted with other administration officials, the Times noted.

Investigators could also seek to question Bannon about Trump’s firing of former FBI Director James Comey, even though he was not present at the time, or the president’s drafting of Trump Jr.’s statement about the June 2016 meeting, where he was also not present. Additionally, he could be interviewed about the president’s decision to fire former National Security Adviser Michael Flynn.

News of the subpoena leaked to the Times as Bannon spoke behind closed doors before the House Intelligence Committee for its Russia probe.

Bannon’s comments in the book prompted the president’s fury, but he has since indicated that his anger may not be permanent.