Tim Morrison, a current high level National Security Council (NSC) official who has been identified as a witness to one of the most explosive pieces of evidence found by House impeachment investigators, will testify next Thursday even if the White House tries to stop him.

“If subpoenaed, Mr. Morrison plans to appear for his deposition,” his attorney, Barbara Van Gelder, said.

Several high-profile witnesses have ignored White House, State Department and Pentagon orders not to cooperate with the impeachment probe. In each case, congressional lawmakers have issued a subpoena, which the officials have used to justify testifying over the administration’s objections.

Morrison, however, would be the first currently serving White House official to testify. He’s also the first official thought to be on the now infamous July 25th phone call between Donald Trump and Ukrainian President Volodymyr Zelensky during which the POTUS seems to pressure the Ukrainian president to dig up dirt on potential 2020 foe, Joe Biden.

Morrison was also a crucial figure named Tuesday by Trump’s ambassador to Ukraine, William Taylor, as a witness to Trump’s effort to not give promised military aid to Ukraine in order to bend Zelensky to his will.

Some additional details from Politico:

Taylor testified that on Sept. 1, Morrison told him about a conversation he witnessed between Trump’s E.U. ambassador Gordon Sondland and a senior Ukrainian government official. In that conversation, Taylor said Morrison told him, Sondland informed the official that hundreds of millions of dollars in U.S. military aid to Ukraine were dependent on the opening of political investigations.

Taylor also said Morrison spoke to him on Sept. 7 about another conversation with Sondland. In that conversation, per Taylor’s recounting, Sondland revealed he had spoken directly with Trump about Ukraine and that Trump insisted that Zelensky should publicly declare the political investigations himself, rather than leaving it to subordinates. According to Taylor, Morrison said he then relayed details of the call to then-national security adviser John Bolton as well as NSC lawyers.

CNN reports that Morrison is expected to back-up Taylor’s version of events, though his attorney declined to confirm that. Other sources, unnamed in the report, say that the NSC official may have testimony that is more nuanced.

It is also interesting to note that Morrison was brought in by John Bolton, and is considered to be a close ally of the former Trump official and nuclear hawk.