President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE on Sunday lashed out on Twitter at special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's investigation, referring to the "13 Angry Democrats" on the probe and writing that they should have been working to investigate Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE controversies.

The Sunday tweet comes as the president has expressed anger and frustration about the FBI's use of an informant in its investigation of Russia's meddling in the 2016 election. Trump and his allies have claimed the use of the informant was an act of politically motivated espionage.

ADVERTISEMENT

"Why didn’t the 13 Angry Democrats investigate the campaign of Crooked Hillary Clinton, many crimes, much Collusion with Russia?" he tweeted. "Why didn’t the FBI take the Server from the DNC? Rigged Investigation!"

Why didn’t the 13 Angry Democrats investigate the campaign of Crooked Hillary Clinton, many crimes, much Collusion with Russia? Why didn’t the FBI take the Server from the DNC? Rigged Investigation! — Donald J. Trump (@realDonaldTrump) May 27, 2018

Trump has used the "13 angry Democrats" term to refer to people on Mueller's probe who he believes are biased against him.

The president, most recently, sent a pair of tweets Saturday night lashing out at those members and casting doubt on Mueller's investigation into reported ties between Trump's campaign and Moscow.

"This whole Russia Probe is Rigged. Just an excuse as to why the Dems and Crooked Hillary lost the Election and States that haven’t been lost in decades. 13 Angry Democrats, and all Dems if you include the people who worked for Obama for 8 years. #SPYGATE & CONFLICTS OF INTEREST!" Trump wrote.

This whole Russia Probe is Rigged. Just an excuse as to why the Dems and Crooked Hillary lost the Election and States that haven’t been lost in decades. 13 Angry Democrats, and all Dems if you include the people who worked for Obama for 8 years. #SPYGATE & CONFLICTS OF INTEREST! — Donald J. Trump (@realDonaldTrump) May 26, 2018

"When will the 13 Angry Democrats (& those who worked for President O), reveal their disqualifying Conflicts of Interest? It’s been a long time now! Will they be indelibly written into the Report along with the fact that the only Collusion is with the Dems, Justice, FBI & Russia?" he added in the second tweet.

When will the 13 Angry Democrats (& those who worked for President O), reveal their disqualifying Conflicts of Interest? It’s been a long time now! Will they be indelibly written into the Report along with the fact that the only Collusion is with the Dems, Justice, FBI & Russia? — Donald J. Trump (@realDonaldTrump) May 26, 2018

Mueller, a lifelong Republican, is in talks with Trump's legal team to set up a potential interview with the president, which could lead to the investigation's conclusion.

The use of the informant predated his appointment, though Trump attorney Rudy Giuliani on Sunday said it still "tainted" Mueller's probe.