Nancy Pelosi has said that a whistleblower's complaint at the centre of Congress's impeachment inquiry shows that US President Donald Trump was engaged in a cover-up.
The House of Representatives speaker read from the declassified complaint, which alleges the White House tried to "lock down" details of Mr Trump's call with the Ukrainian president after he asked him to investigate Democratic presidential candidate Joe Biden.
"This is a cover-up," she said.
The speaker called it a "sad week" in which she changed course and moved forward with a formal impeachment inquiry of the president.
"This is nothing that we take lightly," she said.
Ms Pelosi said the information about the president's call on July 25 with Ukraine's new president and the handling of the complaint "removed all doubt that we should move forward".
The president, she said, "betrayed his oath of office, our national security and the integrity" of our elections.
The complaint alleges that Mr Trump abused the power of his office to "solicit interference from a foreign country" in next year's US election.
The complaint says the White House then tried to "lock down" the information to cover it up, in part by moving it to a different computer system.
Mr Trump has denied doing anything wrong.