On Tuesday, Deputy Assistant Secretary of State George Kent arrived on Capitol Hill to testify.
Three House committees -- the House Committee on Foreign Affairs, Permanent Select Committee on Intelligence, and Committee on Oversight and Reform -- are jointly interviewing Kent.
The Committees are investigating whether President Donald Trump pressed Ukrainian leaders to interfere with the 2020 election. The administration has informed House Democrats that it is refusing to comply with the inquiry, calling it unconstitutional.
The interviews are among what could eventually become dozens of closed-door depositions in the impeachment probe. There are five more scheduled this week, mostly with State Department officials, though it is unclear if they will all appear after Trump declared he wouldn't cooperate with the probe.
The packed schedule of interviews comes as Democrats are methodically working to pin down the details of Trump's pressure on Ukraine. Once Democrats have completed the probe and followed any other threads it produces, they will use their findings to help determine whether to vote on articles of impeachment. Pelosi said she wants the committees to move "expeditiously."