“Mr Volker is an incredible diplomat and a tremendous asset to our country,” Turner said in a statement. “It is deeply unfortunate and regrettable that Schiff’s show trial investigation has clearly affected Volker’s ability to advance U.S. interests with Ukraine.”

The complaint says Volker met in Kyiv with Zelenskiy and other Ukrainian political figures a day after the call and he provided advice about how to “navigate” Trump’s demands.

“I think he was doing the best he could,” said retired senior U.S. diplomat Daniel Fried, who described the actions of his former colleague as trying to guide Ukrainians on “how to deal with President Trump under difficult circumstances.”

Volker’s role, along with Secretary of State Mike Pompeo’s confirmation that he was also on Trump’s July 25 call, deeply entangles the State Department in the impeachment inquiry.

The State Department said Volker has confirmed that he put a Zelenskiy adviser in contact with Trump’s lawyer Rudy Giuliani, at the Ukraine adviser’s request, and Giuliani has said he was in frequent contact with Volker.

“It is my strong belief that Volker would not have been involved in nor permitted anything inappropriate, let alone illegal, in his service to our country.

Separately, The Associated Press reported on Wednesday that Volker met last year with a top official from the same Ukrainian energy firm that paid Biden’s son Hunter to serve on its board. The meeting occurred even as Giuliani pressed Ukraine’s government to investigate the company and the Bidens’ involvement with it.

Pompeo accused the congressional investigators of trying to “bully” and “intimidate” State Department officials with subpoenas for documents and testimony, suggesting he would seek to prevent them from providing information. But the committee managed to schedule the deposition with Volker as well as one next week with Marie Yovanovitch, who was U.S. ambassador to Ukraine until she was removed from the post last spring.

The spotlight is an unlikely place for Volker, who was brought into the Trump administration by Trump’s first secretary of state, Rex Tillerson, to serve as envoy for Ukraine. He worked in a volunteer capacity and had retained his job as head of the John McCain Institute for International Leadership at Arizona State University.

Though his name may not have been known before last week to most Americans, Volker had a long diplomatic career, often working behind the scenes. He was a principal deputy assistant secretary for European and Eurasian affairs before becoming the U.S. ambassador to NATO in 2008.

In his most recent role as envoy to Ukraine, he spoke openly of U.S. support for Ukrainian sovereignty. Last year, he criticized the expansion of Russian naval operations and Russia’s resistance to full deployment of a U.N. peacekeeping mission in eastern Ukraine to monitor the fight against the Russia-backed separatists.

“I do not believe that Volker’s testimony advanced Schiff’s impeachment agenda,” Turner said.

Associated Press reporter Eric Tucker contributed to this report