WASHINGTON, D.C. - The next four key witnesses in the impeachment inquiry were on Capitol Hill Tuesday, including top officials who were on that controversial July phone call between President Donald Trump and the Ukrainian President Volodymyr Zelensky.

Firsthand accounts of the July 25 phone call between Trump and Zelensky headlined Tuesday’s public testimony between two separate hearings, each with two witnesses.

Taking the stand Tuesday morning: Jennifer Williams, a foreign policy aide to Vice President Mike Pence.

“I found the July 25th phone unusual because, in contrast to other presidential calls I observed, it involved discussion about what appeared to be a domestic political matter,” Williams testified.

From the National Security Council -- Lt. Col. Alexander Vindman – the NSC’s top expert on Ukraine – and former senior official Tim Morrison. Former U.S. Special Rep. to Ukraine Kurt Volker also testified during the afternoon hearing.

“When I reported my concerns on July 10threlated to (E.U.) Ambassador (Gordon) Sondland and then July 25related to the President, I did so out of a sense of duty,” said Vindman, whose testimony was highly sought by both Republicans and Democrats.

Vindman said he soon discussed the phone call with State Department official George Kent and another intelligence community official. But Vindman declined to name that person Tuesday. Republicans pressed vindman to learn if that person was the whistleblower, whose tip in September set off the impeachment inquiry. House Intelligence Committee Chairman Adam Schiff (D-Calif.) prompted ended that GOP questioning, saying the committee is not convening to determine the whistleblower’s identity.

“This was a properly cleared individual with a need to know,” Vindman said.

The call still remains the center of this inquiry: whether or not Trump urged Zelensky to investigate former vice president Joe Biden and his son, Hunter, in exchange for approximately $400 million in U.S. military aid. Hunter Biden served on the board of a Ukrainian energy firm, Burisma, while his father was vice president.

In September, less than two weeks before the whistleblower came forward, Ukraine suddenly received the military aid. An investigation into the Bidens was never opened by Ukranian officials.