The Senate Select Committee on Intelligence has issued a subpoena for former national security advisor Michael Flynn, seeking documents in its investigation into Russia's alleged interference in the 2016 presidential election, according to a joint announcement from Sens. Richard Burr and Mark Warner.

Burr and Warner serve as chairman and vice chairman of the committee, respectively.

The Senate panel first requested the documents in an April 28 letter to Flynn, but he refused, through counsel, to turn over the documents.

Late March, the bipartisan leaders of the committee said they were "within weeks" of finishing its review of thousands of documents related to its probe. Burr and Warner said at the time that they wanted to finish the investigation in a "timely" manner, but didn't want to rush it.