WASHINGTON - U.S. President Donald Trump claimed Sunday, without evidence, that the ongoing criminal investigation into his 2016 campaign's links to Russia is "not allowed under the law."

In a Twitter comment, the U.S. leader called the probe by special counsel Robert Mueller "illegal" and said it "continues in search of a crime." But Mueller was appointed by the Department of Justice and judges have ruled that his investigation is being conducted legally.

As he often does, Trump denied there was collusion with Russia, except by his opponent, former Secretary of State Hillary Clinton. He described Mueller's legal team as "17 angry Democrats ... looking at anything they can find. Very unfair and BAD for the country."

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

But Trump did not comment on the latest development in Mueller's 16-month investigation, Friday's guilty plea by former Trump campaign chairman Paul Manafort to corruption charges and a declaration by prosecutors that Manafort is already cooperating with them about what he knows about the Trump campaign.

The 69-year-old Manafort, a longtime Washington lobbyist, has now been convicted in an August trial of eight tax and bank fraud charges in a Virginia court and pleaded guilty to two conspiracy charges in nearby Washington.

FILE - Former Trump campaign manager Paul Manafort FILE - Former Trump campaign manager Paul Manafort arrives for arraignment on charges of witness tampering, at U.S. District Court in Washington, June 15, 2018. FILE - Former Trump campaign manager Paul Manafort arrives for arraignment on charges of witness tampering, at U.S. District Court in Washington, June 15, 2018.

In the most recent case, prosecutors agreed to drop other charges stemming from his lobbying efforts for one-time Ukrainian leader Viktor Yanukovych that predated his five-month tenure working for Trump's campaign in mid-2016 in exchange for him answering "fully, truthfully, completely and forthrightly" questions about "any and all matters" Mueller's team is investigating.

But what Manafort might have to offer Mueller about the Trump campaign is not publicly unknown, although prosecutors only dropped some charges against Manafort after hearing in advance what he had to say.

FILE - People pass police security and a doorman i FILE - People pass police security an a building doorman in front of Trump Tower in New York. FILE - People pass police security an a building doorman in front of Trump Tower in New York.

Manafort played a role in one key event Mueller is investigating, a June 2016 meeting at Trump Tower in New York, the then-candidate's campaign headquarters. Manafort attended a meeting set up by Trump's oldest son, Donald Trump Jr., with a lawyer said to be a Russian government attorney willing to provide incriminating information about Clinton. The younger Trump said no such information was provided and President Trump has said he was unaware of the meeting at the time it was scheduled.

After Manafort's guilty plea, Trump lawyer Rudy Giuliani said in a first statement, "Once again an investigation has concluded with a plea having nothing to do with President Trump or the Trump campaign. The reason: The president did nothing wrong and Paul Manafort will tell the truth."

FILE - President Donald Trump's lawyer Rudy Giulia FILE - President Donald Trump's lawyer Rudy Giuliani speaks to reporters on the South Lawn of the White House in Washington, May 30, 2018. FILE - President Donald Trump's lawyer Rudy Giuliani speaks to reporters on the South Lawn of the White House in Washington, May 30, 2018.

But minutes later, Giuliani issued a revised statement omitting the assessment that "Paul Manafort will tell the truth."

After Manafort was convicted in the August case, Trump said he had "such respect for a brave man: because "he refused to 'break' -- make up stories in order to get a 'deal.'"

"I feel very badly for Paul Manafort and his wonderful family," Trump said.