He’s already said some explosive things about the investigation.

Steve Bannon, former advisor to President Trump, arrives at a House Intelligence Committee closed door meeting, on January 16, 2018 in Washington, DC.

Stephen Bannon, President Donald Trump’s former chief strategist, will now have to divulge what he knows about possible collusion between the Trump campaign and Russia.

The New York Times reports that special counsel Robert Mueller has formally subpoenaed Bannon to testify in front of a grand jury. It’s the first time we know Mueller has used a grand jury subpoena to get a top member of Trump’s inner circle to talk.

Bannon joined the Trump campaign as its chairman in August 2016 and then served as the Trump White House’s top strategist for one year. He would be in a good position to know if the Trump campaign purposefully worked with Russia to sway the 2016 presidential election. However, it’s unclear just how much he might know about other major issues Mueller is surely looking into, including Trump’s firing of former FBI Director James Comey.

A source told the New York Times that it’s possible Mueller’s subpoena is a strategic move designed to convince Bannon to speak with Mueller’s team privately instead of testifying in front of a grand jury.

Regardless, Bannon is an important figure in Trump’s drive to the White House — and he may have interesting things to tell Mueller.

Bannon has already been outspoken about Trump-Russia

Bannon recently lost his job at the helm of Trump-friendly Breitbart in part because of his recent comments about Trump and Russia.

Bannon opined to Michael Wolff, author of the explosive book Fire and Fury: Inside the Trump White House, that Donald Trump Jr.’s meeting with a Russian lawyer in June 2016 in search of dirt on Hillary Clinton was “treasonous.”

He also speculated that then-candidate Trump was possibly involved in the meeting as well, and asserted that Jared Kushner was involved in some “greasy” business that could expose him to money laundering charges. (Trump responded to these allegations with a furious official White House statement denouncing Bannon.)

Bannon also reportedly told Wolff that Mueller’s probe into Trump-Russia could ultimately result in charges related to money laundering.

“This is all about money laundering. Mueller chose [senior prosecutor Andrew] Weissmann first and he is a money-laundering guy. Their path to fucking Trump goes right through [former Trump campaign chair Paul Manafort, [Trump’s son] Don Jr and [Trump’s son-in-law and senior adviser] Jared Kushner,” Bannon is quoted as saying. “It’s as plain as a hair on your face.”

Bannon just so happened to be in a closed-door meeting with the House Intelligence Committee when news of the subpoena broke on Tuesday. It’s possible he’s answering questions about his money laundering allegation during that interview.

Sen. Mark Warner (D-VA), the top Democrat on the Senate Intelligence Committee, also wants to hear what Bannon has to say. “If he’s got facts on [money laundering], I or a special prosecutor ought to hear them,” Warner told me in an interview earlier this month.

It looks like Mueller wants to hear those facts, too.