Special counsel Robert Mueller has subpoenaed Steve Bannon, the former chief strategist for President Trump.

According to the New York Times, Bannon was subpoenaed last week to testify before a grand jury as part of Mueller’s broad probe into Russian meddling in the 2016 presidential election and Russia's possible collusion with the Trump campaign.

Bannon was on Capitol Hill Tuesday testifying behind closed doors before the House Intelligence Committee, which is also investigating Russia’s meddling and possible collusion.

According to the New York Times, this is the first time Mueller has used a grand jury subpoena to get information from someone in Trump’s inner circle.

The subpoena comes nearly immediately after excerpts from Michael Wolff’s book “Fire and Fury: Inside the Trump White House” were published quoting Bannon as calling Donald Trump Jr.’s June 2016 meeting in Trump Tower with the Russians “treasonous.”

Trump and White House officials then denounced Bannon publicly, and also threatened to sue him for defamation.

Bannon, who left the Trump administration in August, stepped down from his post as executive chairman of Breitbart News last week.

Bannon was not directly part of two key situations being probed by Mueller: the firing of FBI director James Comey and the drafting of a misleading statement about the June 2016 meeting. However, according to the New York Times, Bannon’s “secondhand knowledge could be used to draw a contrast with statements from people with firsthand knowledge whom Mr. Mueller has already interviewed.”

Bannon has hired William Burck of a Washington-based law firm to represent him in the defamation threats from Trump, as well as in the congressional investigation.

Burck also represents several current and former administration officials who have been by Mueller’s team, including White House counsel, Donald F. McGahn II and former White House chief of staff Reince Priebus.

Mueller’s probe has already lead him to four members of the Trump campaign and administration.

George Papadopoulos, a Trump campaign foreign policy adviser, pleaded guilty in October to make a false statement to FBI investigators as part of a plea bargain to cooperate with Mueller’s team.

Former national security advisor Michael Flynn also pleaded guilty as part of a plea bargain to "willfully and knowingly" making "false, fictitious and fraudulent statements" to the FBI.

Trump campaign officials Paul Manafort and Rick Gates were indicted in October on nine and eight counts respectively, but have pleaded not guilty — including conspiracy against the U.S. and conspiracy to launder money.