Michael McKinley, who resigned as a senior adviser to US Secretary of State Mike Pompeo last week, is next in line to be questioned in the presidential impeachment inquiry.
On Tuesday, a senior state department official in charge of Ukraine policy testified before the committee.
George Kent was questioned about his knowledge of Trump's efforts to have Ukraine investigate his political rival, former Vice President Joe Biden.
Meanwhile, Democrats are accusing the Trump administration of stonewalling the impeachment inquiry.
Vice President Mike Pence, President Donald Trump's lawyer Rudy Giuliani and the White House are refusing to submit documents.
The head of the House Intelligence Committee, Adam Schiff, says evidence of obstruction of Congress continues to mount.
Michael Isikoff, the chief investigative correspondent at Yahoo News and coauthor of Russian Roulette, talks to Al Jazeera about the developments.