The subpoena letter explicitly cited the Democrats’ newly launched impeachment inquiry into Trump, suggesting that party leaders will lean heavily on impeachment as a legal justification for obtaining disputed documents. A failure to produce them, they warned Pompeo, would be evidence of obstruction.

They gave a deadline of one week for the documents. 

…The lawmakers also notified Pompeo in a separate letter that they had scheduled depositions for five State Department officials between Wednesday and Oct. 10. 

Those officials are Marie Yovanovitch, the former U.S. ambassador to Ukraine; Kurt Volker, a former NATO ambassador who is Trump’s special representative for Ukraine negotiations; State Department Deputy Assistant Secretary George Kent; State Department counselor T. Ulrich Brechbuhl; and Gordon Sondland, U.S. ambassador to the European Union.

Democrats subpoena Pompeo for Ukraine documents | TheHill

hmmmm