FILE PHOTO: Senator Richard Blumenthal questions Supreme Court nominee judge Neil Gorsuch during his Senate Judiciary Committee confirmation hearing on Capitol Hill in Washington Thomson Reuters Democratic Sen. Richard Blumenthal said Tuesday he is "99% sure" that special counsel Robert Mueller's probe into Russian election interference will yield indictments.

Blumenthal, a member of the Senate Judiciary Committee, told Politico on Tuesday that former national security adviser Michael Flynn and former campaign chair Paul Manafort "are the most prominent" subjects of the investigation "but there may well be others" who are vulnerable to indictment, too.

A spokesman for Blumenthal said the senator did not specify in the interview who might be indicted.

Flynn's lawyer and a spokesman for Manafort did not respond to requests for comment.

Blumenthal's comments came as longtime Trump adviser Roger Stone, who has known Manafort for decades, told reporters that Manafort's lawyers had confirmed to his own that Mueller had threatened him with criminal charges following a raid on his Virginia home in late July.

Mueller has since issued subpoenas to a Manafort spokesman, Jason Maloni, and former attorney, Melissa Laurenza, to testify before a federal grand jury. CNN reported last week that the FBI obtained a FISA warrant from the Foreign Intelligence Surveillance Court last year to surveil Manafort.

He was previously monitored under a separate FISA authorization that began in 2014, according to the report, as the FBI scrutinized his lobbying work on behalf of the pro-Russia Party of Regions in Ukraine and his business dealings with Russian entities.

Flynn, meanwhile, came under renewed scrutiny earlier this month when House Democrats sent a letter to Mueller alleging that Flynn had failed to disclose a trip he took to Egypt and Israel in 2015 to pursue a joint Russia-Saudi Arabia project "to build nuclear reactors in the Arab world."

The Wall Street Journal reported shortly afterward that Flynn continued his work on the project when he was at the White House, going as far as to direct his staff at the National Security Council to meet with the companies involved. Ethics advisers on the NSC told Flynn to remove himself from the project, but he continued working on it anyway, according to the Journal.

The trips to Egypt and Israel were not the first foreign contacts Flynn failed to disclose on his security clearance forms. Flynn was paid about $33,000 by Russia's state-owned news agency, Russia Today, for a speech he gave in Moscow in 2015, but he did not disclose the payment on his security clearance application, according to the House Oversight Committee.

Flynn belatedly registered as a foreign agent with the Department of Justice in March for his lobbying work last year on behalf of a Turkish businessman with ties to the Turkish government. Flynn's firm, Flynn Intel Group, was paid about $500,000 by the businessman, Ekim Alptekin, between August and November.

Flynn offered to testify before the congressional intelligence committees in April in exchange for immunity from prosecution — a signal to some legal experts that he thought he may have committed a crime. Neither of the committees has taken him up on the offer.