“The U.S. Congress has a duty to uphold justice, and to take measures to ensure that Special Counsel Mueller’s investigation proceed without interference from the White House,” Pelosi said in a statement on Friday, calling for “an outside, fully independent investigation.” Pelosi has called for an outside investigation for months that could operate in the mold of the 9/11 commission, a bipartisan commission set up by Congress that produced a wide-ranging report on the September 11, 2001, terrorist attack on the World Trade Center.

Adam Schiff, the top Democrat on the House Intelligence Committee, warned in a statement that “the plea secured by Mueller may prompt the White House and its allies to seek to curtail congressional investigations, as President Trump has attempted to do already, or end the special counsel’s work prematurely. Congress must make it clear that this would not be acceptable.” The House and Senate Intelligence Committees are both probing Russian election interference and potential Trump-Russia ties.

Dianne Feinstein, the top Democrat on the Senate Judiciary Committee, similarly said that “investigations must be allowed to continue without interference,” though she did not call for an independent outside investigation.

High-ranking Democrats focused attention on Trump on Friday.

Pelosi called Flynn’s guilty plea “a dark moment in our nation’s history” in her statement, adding that “the American people deserve to know what the President knows about Russian meddling in our election and why he refuses to take action against Russia.”

Schiff added that the Flynn revelations are significant because they include “new details, including that Flynn was directed by a senior transition official to contact Russian officials in an effort” and also because “Flynn’s lies to the FBI … were made at a time when he was serving as President Trump’s National Security Advisor, a position of incredible influence and responsibility.”

Democratic National Committee Chair Tom Perez said “this time, the president can’t get away with claiming these charges aren’t about his inner circle’s contacts with Russia, and he can’t dismiss Michael Flynn as some low-level aide.”

Flynn’s guilty plea marks a new stage in the special counsel investigation.

Last month, Mueller brought charges against three former Trump associates: Former Trump campaign chairman Paul Manafort, Rick Gates, a deputy to Manafort on the campaign, and George Papadopoulos, a former campaign foreign-policy adviser. Trump has called the Russia investigation a “witch hunt.” On Friday, a White House lawyer said that “nothing about the guilty plea [entered by Flynn] or the charge implicates anyone other than Mr. Flynn.”

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.