WASHINGTON - The Senate Intelligence Committee issued a subpoena Wednesday to former National Security Adviser Michael Flynn, the committee's top members announced.

Chairman Richard Burr, a Republican from North Carolina, and Vice Chairman Mark Warner, a Democrat from Virginia, announced Wednesday that the committee has demanded all documents relevant to its investigation into Russian interference with the 2016 election.

The committee had previously requested those documents from Flynn in an April 28 letter, but Flynn, through his counsel, declined, according to Burr. Flynn is also the target of federal grand jury subpoenas, CBS News has confirmed.

The Intelligence Committee's subpoena comes the day after President Trump abruptly fired FBI Director James Comey, claiming Comey was no longer doing a good job. Deputy Attorney General Rod Rosenstein provided the justification for Comey's firing, citing his handling of the Clinton email investigation.

A U.S. official told CBS News Wednesday that Comey had approached Rosenstein last week, asking for more resources to look into any ties between Russia and the president's campaign. The Department of Justice has denied that claim.

Comey was set to testify before the Intelligence Committee Thursday, but Acting FBI Director Andrew McCabe will take his place. The committee has invited Comey to testify in a closed session Tuesday.

Flynn was fired in February, shortly after Mr. Trump took office, after he misled Vice President Mike Pence about his contacts with Russian officials. The timing surrounding Flynn's firing continues to haunt the White House. The White House relied on Obama-era background checks to check Flynn, and former acting Attorney General Sally Yates warned White House officials about Flynn before he was fired.

A January report from the U.S. intelligence community concluded that Russia did indeed meddle in the election, but Mr. Trump has been ambivalent on the issue.

In an interview with CBS' "Face the Nation" last week, the president said it "could've been China" that was responsible for hacking during the campaign.

The president said that "it's very hard" to say who is behind a hack without catching the hacker "in the act."

"With that being said, I'll go along with Russia," Mr. Trump said. "Could've been China, could've been a lot of different groups."

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