The Senate Intelligence Committee has subpoenaed ousted National Security Adviser Michael Flynn for documents related to the committee’s investigation of Russian efforts to interfere in the 2016 presidential campaign.

The subpoena was announced Wednesday by the committee chairman, Sen. Richard Burr (R-N.C.), and ranking member, Sen. Mark Warner (D-Va.).

According to the senators’ joint statement, the committee sought the documents from Flynn last month, but Flynn’s lawyer denied the request. The senators then decided to move forward with a subpoena.

Flynn’s attorney, Robert Kelner, declined to comment on the subpoena.

The Intelligence Committee probe is one of several investigating whether Trump’s campaign colluded with the Russian government to influence the 2016 election.

The committee has also requested documents from Paul Manafort, who served as President Donald Trump’s campaign manager; Roger Stone, who informally advised Trump during his 2016 campaign; and Carter Page, who served as a foreign policy adviser to the campaign, The New York Times reported last week.

Burr on Tuesday threatened to subpoena any Trump campaign officials who did not comply with the committee’s requests.

“I think all options are on the table, and I think you can envision what those options are,” he said. “The most severe would be subpoena.”

Flynn resigned from his post as Trump’s national security adviser after it was revealed he had discussed U.S. sanctions against Russia with that country’s ambassador to the U.S., Sergey Kislyak, prior to Trump’s inauguration.

In March, Flynn reportedly sought immunity in exchange for testifying before the committees investigating potential ties to Russia. Kelner said in a statement at the time that his client “certainly has a story to tell, and he very much wants to tell it, should the circumstances permit.”

“No reasonable person, who has the benefit of advice from counsel, would submit to questioning in such a highly politicized, witch hunt environment without assurances against unfair prosecution,” Kelner said.

The Flynn subpoena came one day after Trump fired FBI Director James Comey, who was overseeing the FBI investigation on alleged collusion with Russia.

This is a developing story and has been updated.