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(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.

Morrison’s testimony also could raise more questions about Sondland’s deposition and his role in pushing Ukraine to do investigations in return for military aid and a meeting with Trump. It’s unclear how many times Sondland and Trump spoke about Ukraine. Morrison did not verify with the President’s secretary the details Sondland had given him of the conversations when he checked whether the calls had occurred.

According to multiple depositions on Capitol Hill, Sondland told Morrison about a call he had with the President in September, in which Trump said he was not asking for a quid pro quo but did want the Ukrainians to announce investigations into former Vice President Joe Biden and his son, Hunter, as well as 2016 foreign election interference, probes that could benefit the President politically.

Morrison, and others who have spoken to Hill investigators, have expressed concerns that Sondland was involved in forming a parallel foreign policy. But Sondland testified that it was Giuliani who was behind the Ukraine investigation push.

In his opening statement, which was obtained by CNN, Morrison contended that in late August he had “no reason” to believe that nearly $400 million in aid to Ukraine might be conditioned on the country announcing an investigation into Burisma, the company that employed Hunter Biden, until he spoke with Sondland on Sepember 1.

“Even then I hoped that Ambassador Sondland’s strategy was exclusively his own and would not be considered by leaders in the administration and Congress, who understand the importance of Ukraine to our national security,” Morrison said to lawmakers.

In his testimony last month, Sondland told lawmakers that he and others had briefed Trump in May about efforts to strengthen relations with Ukraine, but the President had instructed them to deal with Giuliani, something the ambassador said “disappointed” US officials but was a direction that they followed. Sondland later informed lawmakers about a “very short” September phone call with Trump, saying the President was in a “bad mood” as he said repeatedly that there was “no quid pro quo” when asked about the holdup in financial aid to Ukraine.

Sondland’s attorney declined to comment on reports of Morrison’s testimony.

The revelations from Morrison’s testimony are just the latest examples of witnesses contradicting Sondland’s testimony. Earlier this week, Lt. Col. Alexander Vindman – who serves on the White House National Security Council – testified that Sondland had spoken about “specific investigations” at a July meeting in order to secure a bilateral meeting for Ukraine’s President with Trump in Washington, according to a copy of his opening statement. Vindman also testified that Sondland “emphasized the importance that Ukraine deliver the investigations into the 2016 election, the Bidens and Burisma.”

Vindman said he later told Sondland his statements were “inappropriate.”

Yet in Sondland’s own testimony about the July meeting, he denied that anyone “harbored any misgivings about the propriety of what we were doing,” also denying that then-White House official Fiona Hill expressed objections over his comments, contradicting her testimony before House impeachment investigators as well.

Moreover, Sondland testified that he had been unaware that Giuliani’s push to investigate Burisma had anything to do with the Bidens until well after an August phone call he had with the President’s personal attorney.

On Thursday, Morrison also backed up the substance of what Bill Taylor, the top US diplomat in Ukraine, had told lawmakers. During Taylor’s testimony, he laid out the elements of a quid pro quo. Sondland had told a Ukrainian official that the country would not get security assistance until an investigation into Burisma was announced by the Ukraine government, Morrison confirmed.

Morrison was on the phone call in July when Trump asked Ukrainian President Volodymyr Zelensky to look into the Bidens. Morrison told lawmakers that nothing he had heard on the call was illegal, but he was still worried that transcript would leak. Morrison was involved in the discussions over placing the call’s transcript in a separate server, which he admitted was an unusual move, sources told CNN.

Morrison said he was worried about the call leaking out for three main reasons, which did not have legal repercussions. First, he was worried about how it would “play out in Washington’s polarized environment.” Second, he was worried its leak could impact bipartisan support for Ukraine in Congress. Third, he feared the affect the leaked transcript could have on the Ukrainian perceptions of the US-Ukraine relationship.

This story has been updated with additional information.