FBI agents showed up at Steve Bannon’s Washington home last week intent on serving him with a subpoena to appear before a grand jury investigating possible ties between President Donald Trump's campaign and Russia, according to a source familiar with the proceedings.

The agents were unaware at the time that Bannon had retained Washington lawyer William Burck just hours earlier, according to two people familiar with the events that took place on Jan. 9. Once redirected, the agents sent the order to Burck, who is also representing two other witnesses in the probe being led by special counsel Robert Mueller, a former director of the FBI.

Bannon, who served as Trump’s chief strategist until he departed the White House in August, could end up being interviewed by Mueller’s team before the end of the month, according to one source who agreed to discuss the matter on the condition of anonymity.

The subpoena compels Bannon to testify before a grand jury, skipping the voluntary interview with Mueller's team that many in Trump's orbit have elected to take. But Mueller may still leave open the option for an interview in lieu of grand jury testimony. Bannon is likely to accept such an interview if it is made available, according to a source close to Bannon.

Three people familiar with the special counsel’s investigation suggested Mueller moved to subpoena Bannon, rather than ask him to voluntarily appear for questioning, in order to thwart any potential attempt by the White House to pressure Bannon into refusing to cooperate.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Bannon revealed he’d been subpoenaed by the grand jury when he met with lawmakers Tuesday behind closed doors on Capitol Hill, where he was questioned for more than 10 hours.

During the course of questioning, Bannon told members of the House Intelligence Committee that the White House had instructed him not to answer questions related to his tenure in the administration and the transition period, citing a desire to exert executive privilege. The committee responded by issuing a separate subpoena to compel his responses to lawmakers.

Bannon is expected to be more forthcoming with Mueller’s team. “He’ll answer any questions” Mueller wants to ask, one source close to Bannon told NBC News.

Related: Bannon calls Trump Tower Russian meeting ‘treasonous’ in new book

Bannon, who headed the right-wing conservative website Breitbart, was instrumental in Trump’s election and a key figure in the White House. But his abrupt departure from Breitbart last week came amid tensions between the two men that exploded in public after the publication of “Fire and Fury,” a book by Michael Wolff that extensively quoted Bannon criticizing Trump and his family.

In the book, Bannon suggests that Trump knew of, and possibly even met with, Russian agents who had shopped potential dirt on his political rival to his son Donald Trump Jr. and to his campaign chairman at the time, Paul Manafort. The White House has denied that Trump met with any of the Russian participants at that meeting at Trump Tower in June 2016.

According to the book Bannon called the meeting, which also included Jared Kushner, Trump's son-in-law and senior adviser, "treasonous" and "unpatriotic."

After the book’s publication, the White House issued a scathing statement about Bannon, who later expressed regrets for his remarks. But the president’s anger with Bannon led to the loss of his job at Breitbart after the site’s funders sided with the White House.

Rep. Mike Conway, R-Texas, who leads the House Intelligence Committee, said the committee’s subpoena remains in effect and described the interview as in “recess.” It was not clear whether Bannon would appear again before the committee, though Rep. Eric Swalwall, D-Calif., indicated late Tuesday that he was hopeful Bannon would return Thursday.

The White House defended consulting with Bannon's legal representatives prior to Tuesday's closed door hearing.

“As with all congressional inquiries touching upon the White House, Congress must consult with the White House prior to obtaining confidential material,” Press Secretary Sarah Huckabee Sanders said. "We have been fully cooperative with these ongoing investigations, and encourage the committees to work with us to find an appropriate accommodation in order to ensure Congress obtains information necessary to its legitimate interests.”

Burck, who is representing Bannon before the special counsel and the congressional investigations, is also representing White House Counsel Don McGahn and Trump’s former chief of staff, Reince Priebus, both of whom met with Mueller late last year.