The New York Times reports that Special Counsel Robert Mueller has issued a subpoena to former White House chief strategist Steve Bannon to testify before a grand jury as part of the investigation into Russian meddling in the 2016 presidential election.

The subpoena was reportedly issued last week. Bannon is testifying before the House Intelligence Committee today.

In Michael Wolff’s “Fire and Fury,” Bannon accuses Donald Trump Jr., Jared Kushner and former Trump campaign chairman Paul Manafort of essentially betraying the nation by meeting with a group of Russian lawyers and lobbyists who they believed were ready to offer “dirt” on Democratic candidate Hillary Clinton.

More recently, Bannon has said he was not referring to Trump Jr. but rather to Manafort. Wolff stands by his account.

After the book’s release, Trump quickly disavowed “Sloppy Steve Bannon” and argued extensively there was no evidence of collusion between his presidential campaign and operatives tied to Russian President Vladimir Putin. Bannon apologized a few days later, but was stripped of his job leading the pro-Trump news site Breitbart News.

Bannon played a critical role in the campaign, the presidential transition and the White House — all during times now under scrutiny from investigators looking for possible evidence of a connection between Trump’s operations and Russia.