Lt. Col. Alexander Vindman, an Army officer who works for the National Security Council, told the House Intelligence Committee on Nov. 19 that the July call between President Donald Trump and Ukraine's president was "improper." “It is improper for the President of the United States to demand a foreign government investigate a U.S. citizen and political opponent," he said. Vindman and Jennifer Williams, an aide to Vice President Mike Pence, both listened in on a July phone call in which Trump asked the president of Ukraine to investigate former vice president and 2020 presidential candidate Joe Biden and his son, Hunter. In his opening statement, Vindman also said, earlier this year, he became aware of "two disruptive actors" -- Ukraine’s then-Prosecutor General Yuri Lutsenko and Trump's personal lawyer Rudolph Giuliani -- who promoted "false information that undermined the United States’ Ukraine policy."

For more on who’s who in the Trump impeachment inquiry, read: https://www.pbs.org/newshour/politics/whos-who-in-the-trump-impeachment-inquiry