READ: Volker's opening statement to Congress in Ukraine investigation

President TrumpDonald TrumpMeghan McCain: Democrats 'should give a little credit' to Trump for COVID-19 vaccine Trump testing czar warns lockdowns may be on table if people don't get vaccinated Overnight Health Care: CDC details Massachusetts outbreak that sparked mask update | White House says national vaccine mandate 'not under consideration at this time' MORE's former envoy to Ukraine, Kurt VolkerKurt VolkerCNN obtains audio of 2019 Giuliani call linked to Ukraine meddling allegations GOP senators request details on Hunter Biden's travel for probe Yovanovitch retires from State Department: reports MORE, said he was unaware of efforts by the president or his officials to press Ukraine to investigate allegations against former Vice President Joe BidenJoe BidenCDC chief clarifies vaccine comments: 'There will be no nationwide mandate' Overnight Defense: First group of Afghan evacuees arrives in Virginia | Biden signs Capitol security funding bill, reimbursing Guard | Pentagon raises health protection level weeks after lowering it Biden urges local governments to stave off evictions MORE.

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In his opening statement to congressional committees behind closed doors on Thursday, Volker also detailed conversations with Rudy GiulianiRudy GiulianiCapitol insurrection hearing exposes Trumpworld delusions DOJ declines to back Mo Brooks's defense against Swalwell's riot lawsuit Bob Dole: 'I'm a Trumper' but 'I'm sort of Trumped out' 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.