Rep. Devin Nunes, the ranking Republican on the House Intelligence Committee, attempted to turn attention away from evidence related to President Trump’s interaction with his Ukrainian counterpart to questions about the identity of the whistleblower whose complaint triggered the House impeachment investigation and allegations about former Vice President Joe Biden.

The hearing opened with two witnesses in the morning session: Lt. Col. Alexander Vindman, who is on the National Security Council, and Jennifer Williams, a State Department official on loan to Vice President Mike Pence, who both testified they heard Trump attempting to pressure Ukraine President Volodymry Zelensky. Trump has tried to chip away at their credibility, describing them as “never Trumpers.”

Nunes repeatedly asked Vindman about those with whom he discussed Trump’s July 25 phone call with Zelensky. Vindman responded he had been advised not to discuss members of the intelligence committee. The questioning led Committee Chairman Adam Schiff to remind Nunes the hearings would not be used to out the name of the whistleblower.

Earlier, Nunes came out swinging in his opening statement, sarcastically saying if the committee was so interested in bribery, it should be investigating Biden and his son’s position on the board of Ukraine energy company Burisma. In questioning, he asked witnesses if they were aware that Burisma had sent more than $3 million to U.S. accounts tied to Hunter Biden.

Neither Vindman, who speaks both Russian and Ukrainian, nor a second witness, nor Williams said they ever got an explanation of why Trump first withheld and then released $391 million in military aid to Ukraine. The committee is trying to determine whether Trump abused his office, using the aid to pressure Ukraine to do his bidding.

Vindman testified the rough transcript of the July 25 call had a number of omissions, including a reference to Burisma. During the call, Trump asked Zelensky for a “favor,” to investigate the Bidens and a debunked conspiracy theory.

Vindman said he wound up offering Zelensky two pieces of advice: “To be particularly cautious with regard to Russia and its desire to provoke Ukraine, and to stay out of U.S. domestic” politics.

Vindman said he was " concerned” about the July 25 call, which he viewed as a demand rather than a request.

“What I heard was inappropriate. And I reported my concerns to Mr. [John] Eisenberg [the attorney for the National Security Council]. It is improper for the president of the U.S. to demand a foreign government investigate a U.S. citizen and a political opponent," Vindman testified.

Williams said she found the July 25 call “unusual” and gave a copy of the transcript to Pence but did not know whether he had reviewed it prior to a Sept. 1 meeting with Zelensky.

"In contrast to other presidential calls I had observed, it involved discussion of what appeared to be a domestic political matter,” she said, adding, “The reference to Biden sounded political to me.”

The White House blocked Williams from testifying about a subsequent call in September between Trump and Zelensky, but she said she would talk about it in either a classified session or in writing.