President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE's former envoy to Ukraine, Kurt Volker Kurt VolkerGOP senators request details on Hunter Biden's travel for probe Yovanovitch retires from State Department: reports Live coverage: Senators enter second day of questions in impeachment trial MORE, said he was unaware of efforts by the president or his officials to press Ukraine to investigate allegations against former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE.

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In his opening statement to congressional committees behind closed doors on Thursday, Volker also detailed conversations with Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE, the president's personal attorney, and defended Biden from allegations of wrongdoing.

He testified that he attempted to persuade the president and Giuliani against having a negative view on Ukraine, and defended Biden as "a man of integrity."

Volker is one of the first Trump officials to testify in the House impeachment inquiry against Trump. House committees are examining whether Trump sought to gain information on his political rival, Biden, from Ukraine.

Read Volker's opening statement below.