President Trump issued a grave warning Friday to those who allegedly "spied" on his 2016 campaign, calling their actions "treason" and saying “long jail sentences” are in order.

“My Campaign for President was conclusively spied on. Nothing like this has ever happened in American Politics. A really bad situation. TREASON means long jail sentences, and this was TREASON!” Trump tweeted.

Trump's claim that the campaign was "conclusively" spied on follows Attorney General Bill Barr's testimony that "spying did occur" against the Trump campaign in 2016.

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The attorney general is pursuing a formal review into the conduct of that investigation. Earlier this week, it was revealed that Barr had appointed John Durham, the U.S. attorney from Connecticut, to lead that investigation -- to cover “all intelligence collection activities” related to the Trump campaign during the 2016 presidential election and any misconduct during the early stages of the FBI’s original Russia probe.

According to sources familiar with the investigation, Durham has been working on his review of the Russia investigation “for weeks.” He is expected to focus on the period before Nov. 7, 2016—including the use of FBI informants as well as alleged improper issuance of Foreign Intelligence Surveillance Act (FISA) warrants. Durham was asked to help Barr “ensure that intelligence collection activities by the U.S. Government related to the Trump 2016 Presidential Campaign were lawful and appropriate.”

A source also told Fox News that Barr is working “collaboratively” on Durham’s investigation with FBI Director Chris Wray, CIA Director Gina Haspel, and Director of National Intelligence Dan Coats. Durham is also working with Justice Department Inspector General Michael Horowitz, who is currently reviewing allegations of FISA abuses and the role of FBI informants during the early stages of the Russia investigation.

Barr first announced his review, in general terms, on Capitol Hill last month.

“I am reviewing the conduct of the investigation and trying to get my arms around the aspects of the counterintelligence investigation that was conducted in the summer of 2016,” Barr testified on April 9.

The FBI’s July 2016 counterintelligence investigation was opened by former senior agent Peter Strzok. The FBI, at the time, was led by former Director James Comey and former Deputy Director Andrew McCabe—both fired during the Trump administration.

It has been widely reported that in the weeks and months leading up to the 2016 election, the FBI used informants or other investigators to make contact with Trump campaign officials. That issue is part of Durham’s probe, as well as Horowitz’s, which is expected to be completed in the coming weeks.

The latest example was revealed earlier this month, when a New York Times report said that an investigator working for the U.S. intelligence community posed as a Cambridge University research assistant in September 2016, and tried to probe former Trump campaign foreign policy adviser George Papadopoulos on the campaign’s possible ties to Russia.

The investigator, who went by Azra Turk, met with Papadopoulos at a London bar, where she asked directly whether the Trump campaign was working with Russia.

Papadopoulos told Fox News that he saw Turk three times in London: once over drinks, another time over dinner, and then once with Stefan Halper, the Cambridge professor who has been a longtime FBI informant. The Times noted that Turk had apparently been sent to oversee Halper, and possibly to provide cover for Halper in the event Turk needed to testify.

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Papadopoulos told Fox News that Turk was trying to “seduce” him in an effort to “make me slip up and say something that they knew I had no info on.”

Meanwhile, Wray has parted with Barr on the characterization of the FBI's actions as "spying."

“That’s not the term I would use,” Wray told lawmakers on the Senate Appropriations Committee when asked if FBI agents engage in “spying” when they follow FBI policies and procedures.

“Lots of people have different colloquial phrases,” he continued. “I believe that the FBI is engaged in investigative activity, and part of investigative activity includes surveillance activity of different shapes and sizes, and to me the key question is making sure that it’s done by the book, consistent with our lawful authorities.”