The secret whistle-blower complaint at the centre of Congress' impeachment inquiry alleges that President Donald Trump abused the power of his office to "solicit interference from a foreign country" in next year's US election.
The White House then tried to "lock down" the information to cover it up, the complaint said.
The nine-page document was released on Thursday ahead of testimony to House investigators from Joseph Maguire, the acting director of national intelligence, who acknowledged that the complaint alleged serious wrongdoing by the president but insisted that it was not his role to judge whether the allegations were credible or not.
Maguire said he was unfamiliar with any other whistle-blower complaint in American history that "touched on such complicated and sensitive issues."
"I believe that this matter is unprecedented," he said.
More pressure on Trump
The document, with its precise detail and clear narrative, will likely accelerate the impeachment process and put more pressure on Trump to rebut its core contentions and on his fellow Republicans to defend him.
The complaint provides a road map for corroborating witnesses and evidence, which will complicate the president's effort to characterize the findings as those of a lone partisan out to undermine him.
Trump insisted anew that it is all political. After the complaint was released, he immediately tweeted, "The Democrats are trying to destroy the Republican Party and all that it stands for. Stick together, play their game and fight hard Republicans. Our country is at stake." The tweet was in all capital letters.
The whistle-blower complaint centers in part on a July phone call between Trump and Ukrainian President Volodymyr Zelenskiy in which Trump prodded Zelenskiy to investigate Democratic political rival Joe Biden. The White House released a rough transcript of that call on Wednesday.
"In the days following the phone call, I learned from multiple U.S. officials that senior White House officials had intervened to 'lock down' all the records of the phone call, especially the official word-for-word transcript of the call that was produced as is customary by the White House situation room," the complaint says.
The anonymous whistle-blower says that despite him or her not being present for the call, multiple White House officials shared consistent details about it.
White House accused of bid to suppress transcript
Those officials told the whistle-blower that "this was 'not the first time' under this administration that a presidential transcript was placed into this codeword-level system solely for the purpose of protecting politically sensitive — rather than national security-sensitive — information," the complaint said.
The whistleblower said that White House officials had tried to suppress the exact transcript of the call that was produced — as is customary — by the White House Situation Room, according to the complaint.
The officials told the whistle-blower they were "directed" by White House lawyers to remove the electronic transcript from the computer system in which such transcripts are typically stored for coordination, finalisation, and distribution to Cabinet-level officials.
"This set of actions underscored to me that White House officials understood the gravity of what had transpired in the call," the report said.
The whistle-blower said that White House officials had raised concerns that the rough transcript was moved to a separate computer system that is "reserved for codeword-level intelligence information."
The complaint also focuses on Rudy Giuliani, the president's personal lawyer. It says that multiple US officials reported that Giuliani travelled to Madrid one week after the call to meet with one of Zelenskiy's advisers and that the meeting was characterised as a follow-up to the telephone conversation between the two leaders.
White House reaction
House Democrats who are now mulling Trump's impeachment pressed Maguire to explain why he withheld the intelligence community whistle-blower's complaint from Congress for weeks. He insisted he followed the law.
Later in the day, Maguire was to go behind closed doors to speak to the Senate intelligence panel.
House Speaker Nancy Pelosi on Tuesday endorsed an impeachment investigation in light of the Ukraine revelations.
In a statement on Thursday, the White House said, "nothing has changed with the release of this complaint, which is nothing more than a collection of third-hand accounts of events and cobbled-together press clippings all of which shows nothing improper."
Late Wednesday, most Republicans who got an advance look at the complaint were quiet or defended the president as they left secure rooms. But at least one Republican said he was concerned by what he had read.
"Republicans ought not to be rushing to circle the wagons and say there's no 'there there' when there's obviously a lot that's very troubling there," said Nebraska Senator Ben Sasse, a GOP member of the Senate intelligence panel who has been an occasional critic of Trump.
He added that "Democrats ought not be using words like 'impeach' before they knew anything about the actual substance."
Whistle-blower invited to testify
Trump, whose administration had earlier balked at turning over the complaint, said Wednesday afternoon, "I fully support transparency on the so-called whistle-blower information" and that he had communicated that position to House Republican leader Republican Kevin McCarthy of California.
The House and Senate intelligence committees have also invited the whistle-blower to testify, but it is uncertain whether the person will appear and whether his or her identity could be adequately protected without Maguire's blessing.
The unidentified whistle-blower submitted a complaint to Michael Atkinson, the US government's intelligence inspector general, in August.
Maguire then blocked release of the complaint to Congress, citing issues of presidential privilege and saying the complaint did not deal with an "urgent concern."
Atkinson disagreed, but said his hands were tied.