House Speaker Nancy Pelosi said Thursday that a whistleblower's complaint at the center of Congress' impeachment inquiry shows that President Donald Trump was engaged in a cover-up.

Pelosi read from the declassified complaint, which alleges the White House tried to "lock down" details of 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.

Speaker of the House Nancy Pelosi, D-Calif., arrives at the Capitol in Washington, Thursday, Sept. 26, 2019, just as Acting Director of National Intelligence Joseph Maguire is set to speak publicly for the first time about a secret whistleblower complaint involving President Donald Trump. Pelosi committed Tuesday to launching a formal impeachment inquiry against Trump. (AP PhotoJ. Scott Applewhite)

Speaker of the House Nancy Pelosi, D-Calif., arrives at the Capitol in Washington, Thursday, Sept. 26, 2019, just as Acting Director of National Intelligence Joseph Maguire is set to speak publicly for the first time about a secret whistleblower complaint involving President Donald Trump. Pelosi committed Tuesday to launching a formal impeachment inquiry against Trump. (AP PhotoJ. Scott Applewhite)

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.

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."

Speaker of the House Nancy Pelosi, D-Calif., addresses reporters at the Capitol in Washington, Thursday, Sept. 26, 2019, as Acting Director of National Intelligence Joseph Maguire appears before the House Intelligence Committee about a secret whistleblower complaint involving President Donald Trump. Pelosi committed Tuesday to launching a formal impeachment inquiry against Trump. (AP PhotoJ. Scott Applewhite)

Speaker of the House Nancy Pelosi, D-Calif., addresses reporters at the Capitol in Washington, Thursday, Sept. 26, 2019, as Acting Director of National Intelligence Joseph Maguire appears before the House Intelligence Committee about a secret whistleblower complaint involving President Donald Trump. Pelosi committed Tuesday to launching a formal impeachment inquiry against Trump. (AP PhotoJ. Scott Applewhite)

The president, she said, "betrayed his oath of office, our national security and the integrity" of our elections.

The complaint released Thursday morning alleges that Trump abused the power of his office to "solicit interference from a foreign country" in next year's U.S. 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.

Trump has denied doing anything wrong.