The Senate Intelligence Committee issued a subpoena to force former national security adviser Michael Flynn to turn over documents related to the panel’s probe into alleged Russian meddling in the 2016 elections, including possible ties between the Kremlin and the presidential campaigns.

It is the first subpoena the committee has announced in the course of its Russia investigation – a step Chairman Richard Burr, R-N.C., was long reticent to take. But the chairman began signaling this week that if Trump surrogates did not turn over requested materials to the committee by Tuesday – a deadline that some missed – he and Vice Chairman Mark Warner, D-Va., might begin issuing subpoenas.

“Everything has been voluntary up to this point, and we’ve interviewed a lot of people and I want to continue to do it in a voluntary fashion,” Burr said Wednesday morning.

“But if in fact the production of things that we need are not provided, then we have a host of tools,” Burr added, indicating that a subpoena was one of them.

Flynn has come under committee scrutiny over communications he had with Russian Ambassador Sergey Kislyak while a representative of then-President-elect Donald Trump’s transition team. Undisclosed payments that Flynn received from Russian-backed entities, such as Kremlin-sponsored television network RT, have also inspired questions from lawmakers looking into allegations of coordination between the Trump campaign and the Russian government.

Burr and Warner originally requested the documents addressed in the Wednesday subpoena in a letter dated April 28. In announcing the subpoena, the Senate Intelligence Committee said that Flynn “declined, through counsel, to cooperate with the Committee’s request.”