The first witness in the House impeachment inquiry against President Trump told lawmakers Thursday that he warned Rudy Giuliani — Trump’s personal lawyer — that he was getting untrustworthy information from Ukrainian officials about Joe Biden, reports the Washington Post.

“Kurt Volker, who resigned last week after being named in a whistleblower complaint that sparked the House impeachment inquiry of Trump, said he tried to caution Guiliani that his sources … were unreliable and that he should be careful about putting faith in the … [Ukrainian] stories,” the Post says, citing “two people familiar” with Volker’s testimony.

Volker, a former insider though not part of Trump’s inner circle, offered a first-hand account of the Trump administration’s attempt to pressure Ukraine to investigate the former vice president and his son, Hunter.

“Volker appeared straightforward and forthcoming, and there was no question he did not answer, said one person, who spoke on condition of anonymity,” reports the Associated Press.

Giuliani claims that then-vice president Biden led a corrupt plot to head off Ukrainian investigations of a company where Hunter Biden worked. Biden denies the accusation.

During hours of closed-door testimony, the Post says that, according to its sources, “Volker also said that he and other State Department officials cautioned the Ukrainians to steer clear of U.S. politics. Getting involved, he said he told them, would open the nation up to allegations that they were interfering in an election and could be detrimental to Ukraine long-term.”

Republicans who took part in Thursday’s inquiry session shrugged off Volker’s testimony. Rep. Michael Turner (OH) called the whole process a “show trial.”

Along with his oral testimony, Volker turned over dozens of pages of supporting documentation, including photos and “a chain of text messages with Giuliani,” the Post says, citing an unnamed “person familiar with the matter.”

Volker is the first of at least five current and former State Department officials expected to testify to the impeachment panel.