After re-emphasizing that his office would have declared President Donald Trump innocent of obstruction of justice if it had the evidence to do so, special counsel Robert Mueller resigned and formally closed his investigation. Some members of Congress interpreted his nine-minute speech, his first public remarks about the Russia investigation, as a call to action.

“What Robert Mueller basically did was return an impeachment referral. Now it is up to Congress to hold this president accountable,” California Democrat Sen. Kamala Harris tweeted after Mueller’s speech. “We need to start impeachment proceedings. It’s our constitutional obligation.”

In May, Attorney General William Barr testified before the Senate judiciary committee that he believed it was up to Mueller to make decisions on “whether or not conduct was criminal.” On Wednesday, however, Mueller emphasized his office operated under Department of Justice policy, which bars federal criminal charges against a sitting president.

“Charging the president with a crime was therefore not an option we could consider,” he said, reading from prepared remarks. Mueller went on to say that “a process other than the criminal justice system” would be necessary to charge a president with misconduct.

Democratic lawmakers quickly took to Twitter, sure that Mueller had left the ball in their court.

“Mueller’s statement makes clear what those who have read his report know: It is an impeachment referral, and it’s up to Congress to act. They should,” Massachusetts Democrat Sen. Elizabeth Warren said on Twitter.

“Robert Mueller’s statement makes it clear: Congress has a legal and moral obligation to begin impeachment proceedings immediately,” New Jersey Democrat Sen. Cory Booker tweeted.

While Democrats seemed to be gearing up for an impeachment battle, Republican legislators put out statements asserting Mueller’s remarks revealed nothing new.

“Today’s statement by Mr. Mueller reinforces the findings of his report. And as for me, the case is over,” tweeted South Carolina Sen. Lindsey Graham. “Mr. Mueller has decided to move on and let the report speak for itself. Congress should follow his lead.”

“Special Counsel Mueller confirmed today what we knew months ago when his report was released: There was no collusion and no obstruction,” tweeted Georgia Republican Rep. Doug Collins. “Relitigating the 2016 election and reinvestigating the special counsel’s findings will only further divide our country.”

The Republicans reflected the views of the president, who tweeted his own statement out shortly after Mueller concluded his remarks.

“Nothing changes from the Mueller Report,” he said. “There was insufficient evidence and therefore, in our Country, a person is innocent. The case is closed!”

For more on the origins of the special counsel’s probe into Russian interference in the 2016 election, watch The Mueller Investigation.