The US ambassador to the European Union, Gordon Sondland, told Congress that President Donald Trump's Ukraine efforts were a quid pro quo, Sondland's lawyer told The Wall Street Journal.

Sondland testified that he believed the Trump administration swapped a White House meeting for a promise to investigate former Vice President Joe Biden, his son, and 2016 election interference.

Sondland also told lawmakers that he believed Trump's decision to withhold $400 million in aid to Ukraine in exchange for the investigations was a quid pro quo arrangement, though he added that he was not a lawyer.

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The US ambassador to the European Union told House impeachment investigators that President Donald Trump's efforts to have Ukraine investigate the Bidens were a quid pro quo, the ambassador's lawyer told The Wall Street Journal.

Gordon Sondland testified last week that he believed the Trump administration had exchanged a White House meeting with Ukraine's president for investigations into former Vice President Joe Biden's son and 2016 election interference, Sondland's lawyer, Robert Luskin, told The Journal.

Lawmakers also asked Sondland whether Trump's decision to withhold $400 million in aid was part of a quid pro quo arrangement for Ukrainian officials to investigate Biden, Sondland responded that he wasn't a lawyer but believed it was a quid pro quo, Luskin said.

Sondland testified as part of a House impeachment inquiry, which has been investigating whether Trump used the office of the president for his own personal gain.

A major facet of the investigation is Trump's withholding of the military aid package Congress had allocated for Ukraine, and whether Trump sought to exchange it for the investigation into Biden.

Ukraine's President Volodymyr Zelenskiy meets US President Donald Trump at the United Nations General Assembly in New York City on September 25, 2019. REUTERS/Jonathan Ernst

Trump and his personal attorney, former New York City Mayor Rudy Giuliani, had been pressuring Ukraine to investigate corruption allegations against the Bidens, as well as the US intelligence community's assessment that Russia meddled in the 2016 election.

Sondland, along with several other diplomats, were called to testify over their now-infamous text conversations regarding the White House's efforts to mount pressure on Ukraine.

One September 9 text exchange in particular — between Sondland and the acting ambassador to Ukraine, William Taylor — has become a key focus of the House impeachment inquiry.

"I think it's crazy to withhold security assistance for help with a political campaign," Taylor texted Sondland.

Roughly five hours later, after reportedly phoning Trump, Sondland responded to Taylor.

"The President has been crystal clear: no quid pro quo's of any kind," Sondland wrote. "The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelenskiy promised during his campaign."

Trump himself has denied a quid pro quo arrangement.

Sondland's lawyer told The Journal that Sondland testified that he wasn't involved in Trump's decision to withhold the Ukraine aid, and could not prove that the decision wasn't related to the demand for investigations.