WASHINGTON (AP) — The former special envoy to Ukraine testified Tuesday he should have realized — as many of his colleagues did — that President Donald Trump was holding up military aid to pressure the country to investigate political rival Joe Biden.

Testifying in the House impeachment inquiry, Kurt Volker said he believes now, thanks to hindsight and the testimony of other witnesses, that Trump was using the aid to compel Ukraine to investigate Biden and his son Hunter, who was on the board of a Ukrainian gas company, Burisma.

But Volker insisted he did not know of the effort at the time, despite his deep involvement with Ukrainian officials on a statement — never released — that would have committed the country to investigating Burisma and the 2016 U.S. election.

Nor did he make the connection after Rudy Giuliani, Trump's personal lawyer, mentioned allegations against Joe Biden during a July 19 breakfast, Volker said.

"In retrospect I should have seen that connection differently, and had I done so, I would have raised my own objections," he said Tuesday.

Volker was testifying alongside former White House national security official Tim Morrison in the second hearing of the day in the House's impeachment inquiry, the fourth in history against a U.S. president. Both witnesses were requested by Republicans.

Democrats say there may be grounds for impeachment in Trump's push for Ukraine's new leader to investigate his Democratic rival and the 2016 U.S. election as he withheld critical U.S. military assistance.

Trump denies any such quid pro quo, and he dismissed the hearings Tuesday as a "kangaroo court."

Morrison, who stepped down from the National Security Council shortly before he appeared before House investigators behind closed doors last month, has said he was not concerned that anything illegal was discussed on Trump's July 25 call, something Republicans have repeatedly highlighted.

"As I stated during my deposition, I feared at the time of the call on July 25th how its disclosure would play in Washington's political climate," he said Tuesday. "My fears have been realized."

He told lawmakers Tuesday that the transcript of the call was incorrectly placed in a highly secure location.

"It was a mistake," he said, merely "an administrative error."

Volker was the first person to testify behind closed doors in the inquiry that started in September, resigning his position shortly before he did so.

Since then, a parade of witnesses have testified publicly and privately about what they recalled about the July 25 phone call between Trump and Ukraine's new leader, Volodymyr Zelenskiy. Many of those statements cast doubt on Volker's account that he didn't know Burisma was tied to Biden, and that he wasn't aware of a possible quid pro quo. Volker was not on the call.

On Tuesday, Volker said he opposed any hold on security assistance.

"I did not understand that others believed that any investigation of the Ukrainian company, Burisma, which had a history of accusations of corruption, was tantamount to investigating Vice President Biden," he said. "I drew a distinction between the two."

Even though, he said, he understood that Hunter Biden had been a board member.

Morrison has confirmed to investigators that he witnessed a key September conversation in Warsaw between Gordon Sondland, the U.S. ambassador to the European Union, and a Ukrainian official. Sondland told the official that U.S. aid might be freed if the country's top prosecutor "would go to the mike and announce that he was opening the Burisma investigation," Morrison said in previous closed-door testimony.

Volker shifted his account of a pivotal July 10 interaction at the White House. In his closed-door interview last month, he said there was no discussion of Giuliani's activities in Ukraine or investigations sought by the president.

But on Tuesday, he said the meeting was essentially over when Sondland made a "general" comment about investigations.

"I think all of us thought it was inappropriate; the conversation did not continue and the meeting concluded," Volker said.

A series of text messages Volker provided to lawmakers showed conversations between him, Sondland and other leaders where they discuss the need for Ukraine to launch investigations, including into Burisma.

Volker said meeting with Giuliani was just part of the dialogue, and he had one in-person meeting with him, in which Giuliani "raised, and I rejected, the conspiracy theory that Vice President Biden would have been influenced in his duties as Vice President by money paid to his son."

He said he has known Biden for more than two decades and believes him to be an honorable man.

"The allegations against Vice President Biden are self-serving and non-credible," Volker said.

Volker said he wasn't part of an irregular foreign policy channel led Giuliani, as others have testified. He also said Trump never christened him, Ambassador Gordon Sondland and Energy Secretary the "three amigos" in charge of Ukraine policy.

"My role was not some irregular channel, but the official channel," Volker said.

Volker also said a senior aide to Zelenskiy approached him last summer to ask to be connected to Giuliani. He said he made clear to the Zelenskiy aide, Andrey Yermak, that Giuliani was a private citizen and not a representative of the U.S. government.

Volker himself requested a meeting in July with Giuliani. He said Giuliani mentioned the accusations about the Bidens, as well as a discredited theory that Ukraine interfered in the 2016 U.S. election.

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Associated Press Writers Mary Clare Jalonick, Matthew Daly, Alan Fram, Lisa Mascaro and Laurie Kellman contributed to this report.