Washington — In extraordinary testimony that left lawmakers stunned, Bill Taylor — the top U.S. diplomat in Ukraine — testified on Tuesday that U.S. aid to Ukraine was explicitly tied to the country's willingness to investigate President Trump's political rivals, providing new details about the events at the center of the impeachment inquiry.

Taylor, the chargé d'affaires of the U.S. embassy in Kiev, delivered a 15-page statement behind closed doors to members of the House committees leading the impeachment probe. Taylor emerged as a key witness in the investigation into the Trump administration's efforts to pressure Ukraine when text messages between him and two other top diplomats came to light earlier this month.

In the statement, Taylor describes a concerted effort to use U.S. leverage to get Ukrainian President Volodymyr Zelensky to commit to opening investigations into debunked allegations of Ukrainian interference in the 2016 U.S. election, as well as the gas company Burisma, which had hired former Vice President Joe Biden's son in 2014. Taylor said these efforts came via an "irregular, informal channel of U.S. policy-making" consisting of Rudy Giuliani, then-special envoy Kurt Volker, Energy Secretary Rick Perry and U.S. Ambassador to the E.U. Gordon Sondland.

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CBS News obtained a copy of Taylor's testimony, which was first published by The Washington Post.

Taylor's interactions with Sondland in particular shed new light on the events at the center of the Ukraine scandal. Taylor told lawmakers Sondland "said that President Trump wanted President Zelenskyy 'in a public box' by making a public statement about ordering such investigations." He said he slowly came to realize that a delay in U.S. military aid to Ukraine was explicitly tied to the Ukrainians' willingness to state publicly that it would launch investigations into 2016 and the Bidens, which he described as "extremely troubling."

Read Taylor's full opening statement here:

Taylor's arrival in Ukraine

Taylor, a West Point grad and public servant for 50 years, became the top diplomat in the Kiev embassy in May after the ouster of Marie Yovanovitch, the former ambassador who was recalled three months early for a perceived lack of loyalty to the Trump administration. Yovanovitch was publicly vilified by Trump allies, including Giuliani and Donald Trump Jr., and the State Department's treatment of her left Taylor hesitant to accept the assignment.

"I cared about Ukraine's future and the important U.S. interests there. So, when Secretary Pompeo asked me to go back to Kyiv, I wanted to say 'yes,'" Taylor said. "But it was not an easy decision. The former Ambassador, Masha Yovanovitch, had been treated poorly, caught in a web of political machinations both in Kyiv and in Washington. I feared that those problems were still present. When I talked to her about accepting the offer, however, she urged me to go, both for policy reasons and for the morale of the embassy."

Taylor said he was "worried" about Giuliani's involvement in U.S. policy toward Ukraine, and told Secretary of State Mike Pompeo that he would only take the position if the U.S. continued to back the Ukrainian government.

"I made clear to him and the others present that if U.S. policy toward Ukraine changed, he would not want me posted there and I could not stay. He assured me that the policy of strong support for Ukraine would continue and that he would support me in defending that policy," Taylor said.

Taylor arrived in the country several weeks later and said he "discovered a weird combination of encouraging, confusing, and ultimately alarming circumstances." He said he was encouraged by Zelensky's early moves in office, particularly the appointment of reformists into government posts and moves to strengthen protections against corruption among public officials.

But Taylor also found parallel tracks of U.S. policy-making: an official channel that he was responsible for, and another "highly irregular" operation comprising Sondland, Volker, Perry and Giuliani. Taylor said Volker, Sondland and Perry briefed the president after attending Zelensky's inauguration, encouraging him to meet with the Ukrainian leader "to cement the U.S.-Ukraine relationship."

"From what I understood, President Trump did not share their enthusiasm for a meeting with Mr. Zelensky," Taylor said.

"The President 'wanted to hear from Zelenskyy'"

At first, the goals of the two diplomatic channels aligned around arranging a meeting between the president and Zelensky, but they soon began to diverge.

"[D]uring my subsequent communications with Ambassadors Volker and Sondland, they relayed to me that the President 'wanted to hear from Zelenskyy' before scheduling the meeting in the Oval Office," Taylor said. "It was not clear to me what this meant."

Taylor said he spoke to Sondland on June 27, and Sondland said Zelensky needed to clarify to Mr. Trump that he "was not standing in the way of 'investigations.'" Taylor also said he was caught off guard by Sondland excluding other U.S. officials in a call with Sondland, Perry, Volker, Taylor and Zelensky the next day.

Before Zelensky joined the call, Volker told his colleagues that "he would relay that President Trump wanted to see rule of law, transparency, but also, specifically, cooperation on investigations to 'get to the bottom of things.'" Taylor said he reported the call to Washington and wrote a memo summarizing it two days later.

"By mid-July it was becoming clear to me that the meeting President Zelenskyy wanted was conditioned on the investigations of Burisma and alleged Ukrainian interference in the 2016 U.S. elections," Taylor wrote. "It was also clear that this condition was driven by the irregular policy channel I had come to understand was guided by Mr. Giuliani."

On a conference call on July 18, Taylor said he heard an official from the White House Office of Management and Budget (OMB) say that there was a hold placed on hundreds of millions of dollars in military aid to Ukraine for use in fending off Russian aggression. The official said the order to put the aid on hold came "from the President to the Chief of Staff to OMB," Taylor wrote.

"In an instant, I realized that one of the key pillars of our strong support for Ukraine was threatened," he said. "The irregular policy channel was running contrary to goals of longstanding U.S. policy."

It was around this time, Taylor said, that he "began to sense that the two decision making channels — the regular and the irregular — were separate and at odds."

"I will leave no stone unturned"

Ahead of the now-infamous July 25 phone call between Mr. Trump and Zelensky, Taylor said Sondland and Volker stressed the importance of getting the Ukrainian leader to commit to "investigations."

"Ambassador Sondland told me that he had recommended to President Zelenskyy that he use the phrase, 'I will leave no stone unturned' with regard to investigations," Taylor said. When the two leaders spoke, Taylor said he "strangely" did not receive a readout of the conversation.

In a meeting with Zelensky and Volker the next day, the Ukrainian president said "he was happy with the call but did not elaborate." He then asked about a visit to the White House that Mr. Trump had floated in a letter Taylor delivered to Zelensky in May.

Taylor wrote that he was prepared to resign by mid-August over the delay in U.S. aid. He said Tim Morrison, the official in charge of the Eurasia desk at the National Security Council (NSC), told him the president "doesn't want to provide any assistance at all," which Taylor found "extremely troubling."

"As I had told Secretary Pompeo in May, if the policy of strong support for Ukraine were to change, I would have to resign. Based on my call with Mr. Morrison, I was preparing to do so," Taylor wrote.

Connecting military aid to "investigations"

On August 27, Taylor met with then-National Security Adviser John Bolton in Kiev and expressed his concerns about the delay in aid. Bolton told him to relay his misgivings in a cable directly to Pompeo, which Taylor said he did on August 29. He did not hear back, but said he learned Pompeo had later taken his cable to a White House meeting.

Politico reported that day the Trump administration had placed the military aid package on hold, the first time the delay spilled out into public. Taylor said an aide to Zelensky contacted him and was "very concerned." At that point, Taylor said he had not yet connected the delay in aid to the pressure on Ukraine to open "investigations," but he soon would.

Morrison, the NSC official, told Taylor that Sondland, the E.U. ambassador, had told the Zelensky aide that the military assistance funds would not be released until Zelensky committed to investigating Burisma, the energy company that employed Hunter Biden.

"This was the first time I had heard that the security assistance — not just the White House meeting — was conditioned on the investigations," Taylor wrote.

That same day, Taylor messaged Sondland to ask whether "security assistance and [a ] WH meeting are conditioned on investigations?" Sondland and Taylor then spoke on the phone, with Sondland saying that Mr. Trump told him he wanted Zelensky to publicly commit to investigating both alleged interference in 2016 and Burisma.

"'Everything' was dependent on such an announcement, including security assistance. [Sondland] said that President Trump wanted President Zelenskyy 'in a public box' by making a public statement about ordering such investigations," Taylor said. He urged Sondland to push back on Mr. Trump's attempts to pressure Zelensky, and Sondland said he would try.

"The push to make President Zelenskyy publicly commit to investigations of Burisma and alleged interference in the 2016 election showed how the official foreign policy of the United States was undercut by the irregular efforts led by Mr. Giuliani," Taylor said.

A "stalemate"

On September 7, Taylor spoke to Morrison, the NSC official. Morrison described a call between Mr. Trump and Sondland, and said the president told the ambassador he was not seeking a "quid pro quo."

"But President Trump did insist that President Zelenskyy go to a microphone and say he is opening investigations of Biden and 2016 election interference, and that President Zelenskyy should want to do this himself," Taylor said.

The next day Taylor spoke to Sondland, who reiterated the president's insistence he was not pursuing a quid pro quo. Nonetheless, Sondland said the president was adamant that Zelensky had to "clear things up and do it in public." Sondland said he relayed that to Zelensky and his aide, warning them that if they did not "clear things up" then they would be at a "stalemate."

"I understood a stalemate to mean that Ukraine would not receive the much-needed military assistance," Taylor said. "Ambassador Sondland said that this conversation concluded with President Zelenskyy agreeing to make a public statement in an interview with CNN."

Taylor followed up with a text message to Sondland saying that his "nightmare is they (the Ukrainians) give the interview and don't get the security assistance. The Russians love it. (And I quit.)."

The following day, September 9, Taylor wrote to both Volker and Sondland expressing his strongest reservations yet. "I think it's crazy to withhold security assistance for help with a political campaign," he wrote.

Sondland called him five hours later, telling him he was "incorrect" about the president's intentions and reiterating that he said there would be no quid pro quo.

"The explanation made no sense"

Taylor recounts conversations with Sondland and Volker, who both deployed similar anecdotes to explain the president's behavior.

"Ambassador Sondland tried to explain to me that President Trump is a businessman. When a businessman is about to sign a check to someone who owes him something, he said, the businessman asks that person to pay up before signing the check," Taylor said, adding that Volker said something similar several days later. "I argued to both that the explanation made no sense: the Ukrainians did not 'owe' President Trump anything, and holding up security assistance for domestic political gain was 'crazy.'"

The delay in aid was eventually lifted, and the money was released on September 11. Taylor received assurances from Zelensky's office that he would not give CNN an interview to announce the investigations.

He did not learn details of the president's July 25 phone call with Zelensky until September 25, when the White House released a summary of the call.

"Although this was the first time I had seen the details of President Trump's July 25 call with President Zelenskyy, in which he mentioned Vice President Biden, I had come to understand well before then that 'investigations' was a term that Ambassadors Volker and Sondland used to mean matters related to the 2016 elections, and to investigations of Burisma and the Bidens," Taylor wrote.

The longtime U.S. diplomat left Capitol Hill shortly before 8 p.m. Tuesday night, after 9.5 hours of testimony.