President Trump marked the first anniversary of the special counsel’s Russia investigation Thursday by repeating accusations that the FBI had a confidential informant inside his 2016 campaign and by calling the probe “disgusting, illegal and unwarranted.”

Trump tweeted: “Wow, word seems to be coming out that the Obama FBI ‘SPIED ON THE TRUMP CAMPAIGN WITH AN EMBEDDED INFORMANT.’ Andrew McCarthy says, ‘There’s probably no doubt that they had at least one confidential informant in the campaign.’ If so, this is bigger than Watergate!”

Trump appeared to be reacting to recent news reports that there may have been a top-secret source inside the Trump campaign providing intelligence to the FBI as it began its investigation into Russia’s interference in the election process.

But claims by Trump allies that there was an informant embedded inside the campaign have not been substantiated.

Senate Republicans don’t want President Trump to fire special counsel Robert S. Mueller III. They just won't protect Mueller if Trump decides to fire him. (JM Rieger/The Washington Post)

Andrew McCarthy, a former federal prosecutor, appeared on “Fox & Friends,” the Fox News Channel morning show that Trump regularly watches, shortly before the president’s tweet speculating about FBI informants inside Trump’s campaign.

Trump, his attorney Rudolph W. Giuliani and White House officials have seized on the matter in an effort to undermine the probe of special counsel Robert S. Mueller III.

In a second Thursday morning tweet, Trump said: “Congratulations America, we are now into the second year of the greatest Witch Hunt in American History . . . and there is still No Collusion and No Obstruction. The only Collusion was that done by Democrats who were unable to win an Election despite the spending of far more money!”

And in a third tweet, Trump said: “Despite the disgusting, illegal and unwarranted Witch Hunt, we have had the most successful first 17 month Administration in U.S. history — by far! Sorry to the Fake News Media and ‘Haters,’ but that’s the way it is!”

Senate Minority Leader Charles E. Schumer (D-N.Y.) responded to Trump’s tweets in a speech from the Senate floor Thursday.

“I would say to the president, it’s not a ‘witch hunt’ when 17 Russians have been indicted,” Schumer said. “It’s not a ‘witch hunt’ when some of the most senior members of the Trump campaign have been indicted. It’s not a ‘witch hunt’ when Democrats and Republicans agree with the intelligence community that Russia interfered in our election to aid President Trump. Any fair-minded citizen, even the most ardent partisan, should be able to look at the facts and say that this investigation is not a ‘witch hunt.’”

Trump’s lawyers have been in negotiations with Mueller’s team for months about whether the president would sit for an interview.

Giuliani told The Washington Post and other news organizations Wednesday that the special counsel’s team informed the president’s attorneys recently that prosecutors do not believe they can charge a sitting president with a crime under Justice Department guidelines, signaling they would leave it to Congress to address any possible findings of wrongdoing by Trump.

During a Thursday interview on “Fox & Friends,” Giuliani, a former New York mayor, called for an end to the Mueller probe.

“I think the investigation should be thrown out,” Giuliani said. Going on to comment specifically on the allegations of an FBI informant, Giuliani said he was “shocked to hear that they put a spy in the campaign of a major-party candidate, maybe two spies.”

He added: “We’re going to have to look into whether we can challenge the legitimacy of the entire investigation. Maybe a special counsel, special prosecutor, never should have been appointed.”

Kellyanne Conway, Trump’s 2016 campaign manager who now is a White House adviser, also appeared on “Fox & Friends” Thursday and said: “It looks like the Trump campaign in fact may have been surveilled. It looks like there was an informant there. As the president likes to say, we’ll see what happens.”

The top-secret intelligence source has been the subject of a showdown between key Trump allies in Congress and the Justice Department.

House Intelligence Committee Chairman Devin Nunes (R-Calif.) has sought classified documents from the Justice Department, saying he needs to review the records as part of his congressional oversight duties.

But, as The Post reported earlier this month, senior FBI and national intelligence officials notified Nunes that doing so could endanger a top-secret intelligence source and risk lives.