The Senate Intelligence Committee on Wednesday issued a subpoena to former National Security Adviser Michael Flynn for documents pertaining to the ongoing investigation of Russian meddling in the U.S. election.

The committee announced it was issuing the subpoena after Flynn declined a voluntary request for cooperation.

Chairman Richard M. Burr of North Carolina and Vice Chairman Mark Warner of Virginia had requested documents from Flynn on April 28, according to the notification from the committee.

Speaking with reporters earlier in the day, Burr said he preferred when people cooperate, but pointed to his willingness to use the power of the committee to compel document production.

“We’ve interviewed a lot of people, and I want to continue to do it a voluntary fashion, but if in fact the production of things that we need are not provided, then we have a host of tools,” Burr said.

The Intelligence panel has extended an invitation to ousted FBI Director James B. Comey to appear before it in a closed session next Tuesday.