President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE on Sunday reiterated his claim that 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 into possible collusion between his campaign and Russia during the 2016 election is illegal.

"The illegal Mueller Witch Hunt continues in search of a crime," Trump tweeted Sunday morning. "There was never Collusion with Russia, except by the Clinton campaign, so the 17 Angry Democrats are looking at anything they can find."

"ALSO, not allowed under the LAW!" Trump added.

The illegal Mueller Witch Hunt continues in search of a crime. There was never Collusion with Russia, except by the Clinton campaign, so the 17 Angry Democrats are looking at anything they can find. Very unfair and BAD for the country. ALSO, not allowed under the LAW! — Donald J. Trump (@realDonaldTrump) September 16, 2018

Trump's comment comes just two days after his former campaign chairman Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE pleaded guilty to two federal charges and reached a deal to cooperate with Mueller and avoid a second trial on additional charges in Washington, D.C.

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Manafort, who is the fourth former Trump associate to reach a cooperation agreement with Mueller, said he will cooperate "fully and truthfully” with the special counsel, according to the deal.

Trump has regularly denounced Mueller's investigation as a "witch hunt" driven by political motivations to undermine his presidency.

On Thursday, he called the investigation "illegal" in an interview with Bloomberg News, pointing to "great scholars" who said "there never should have been a special counsel."

Speculation has mounted over whether Trump may attempt to fire Mueller or his Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE, though the president has been advised against doing so.

Former independent counsel Ken Starr said Saturday that there would be "hell to pay" politically for Trump if he were to fire Sessions and Mueller. Starr served on the 1998 investigation of former President Clinton, who was eventually impeached by the House but not convicted by the Senate on charges of perjury and obstruction of justice.

Trump, in recent weeks, has increasingly turned to direct calls for Sessions to end the investigation.

"Attorney General Jeff Sessions should stop this Rigged Witch Hunt right now, before it continues to stain our country any further," Trump tweeted in early August.

The tweet drew immediate backlash from Trump critics, who view it as a potential example of obstruction of justice. Mueller is reportedly examining Trump's tweets for potential obstruction of justice charges.