(CNN) A top White House official told lawmakers he tried to find out whether President Donald Trump told a key US diplomat he wanted Ukraine to investigate the Bidens, multiple sources familiar with his closed-door impeachment inquiry deposition on Capitol Hill told CNN.

His actions show concern inside the White House about the extent of the President's role in the push for investigations that could help Trump politically.

Tim Morrison, the President's top Russia adviser, had multiple conversations with American Ambassador to the European Union Gordon Sondland. In those discussions, the ambassador referenced talks he had with the President. Morrison became concerned that Sondland was going rogue on Ukraine.

Morrison told lawmakers he thought Sondland was a "free radical," according to two of the sources. The term was a reference to molecules that cause cancer.

To find out whether Sondland had talked to the President, Morrison went so far as asking Trump's executive secretary if the President had actually talked with Sondland. The ambassador's claims about having the conversations checked out each time, Morrison said in his testimony Thursday, according to the sources. In his own opening statement, Sondland downplayed both Trump's role and his own in the effort to pressure Ukraine -- suggesting he was reluctantly working with Rudy Giuliani, the President's personal attorney, who was running a shadow diplomatic operation in Ukraine.

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