U.S. President Donald Trump has granted Attorney General William Barr new sweeping powers to investigate Trump's claim that U.S. intelligence agencies engaged in treasonous "spying" on his 2016 election campaign.

Trump ordered intelligence agencies on Thursday to "promptly provide" all material that Barr seeks in the attorney general's pledge to review the investigations of the 2016 election.

The directive, which strips the heads of intelligence agencies of their command over classified documents, delighted Trump allies like Sebastian Gorka. In a video released Thursday, the former Trump campaign aide proclaimed: "The Kraken has been released," referencing the mythical sea monster.

Barr now has the authority to pick and choose material to declassify in his inquiry into surveillance of Trump's campaign, raising fears selective revelations could tell a politically slanted story.

House Intelligence Committee Chairman Adam Schiff, a Democrat, blasted Trump for trying to "weaponize" classified information to use against his political foes.

"Barr obviously got a view of his job that has to do with being a political partisan for the president," said Harry Litman, a former U.S. deputy assistant attorney general. "A good way to do that would be to take good little snippets and ... reveal them to tell a certain narrative.

"The power is worrisome."

But what's behind the president's allegation of illegal "spying" in the first place?

Where does Trump's spying claim come from?

Former Trump campaign associate Carter Page, shown Dec. 12, 2016, was the subject of a court order monitoring his communications. (Pavel Golovkin/The Associated Press)

Mainly, the accusation references FBI intelligence-gathering on two Trump campaign associates — former Trump campaign adviser Carter Page and former Trump foreign policy adviser George Papadopoulos — in 2016.

The FBI intercepted Page's electronic communications. The bureau also used a confidential informant and an undercover investigator to reach out to Papadopoulos. Both were initially investigated due to suspected links with Russian operatives.

What does Barr say about this?

In April, Barr testified before Congress that he believed "spying did occur" on the Trump campaign during the 2016 race.

Barr's probe has become known in Washington as an "investigation into the investigators," a reference to Trump's demands to look into what gave rise to U.S. Special Counsel Robert Mueller's report on Russian election interference.

Critics have pointed out that Barr should be familiar with the precise parlance of intelligence officers given his years serving as counsel in the CIA. But Barr defended his use of the word "spying" as "a good English word."

"I don't think the word 'spying' has any pejorative connotation at all," he said.

What is Trump saying?

U.S. Attorney General William Barr, right, who was confirmed into the position three months ago, has been given wide latitude by Donald Trump to probe what the president sees as a politically motivated investigation that began in 2016. (Carlos Barria/Reuters)

After Barr's testimony last month, the president doubled down.

"There was absolutely spying into my campaign," he said. "I'll go a step further. In my opinion, it was illegal spying, unprecedented spying, and something that should never be allowed to happen in our country again."

Trump has also accused FBI officials of "treason" without evidence.

On Thursday, Trump defended his move to give Barr authority to reveal secrets gathered by intelligence agents in the Russia probe, saying the directive was necessary "so everything that they need is declassified, and they'll be able to see how this hoax — how the hoax or witch hunt — started and why it started."

Trump told reporters he believes the surveillance was part of "an attempted coup" against his presidency.

What did the FBI do?

Former Trump campaign adviser George Papadopoulos pleaded guilty for lying to investigators in U.S. Special Counsel Robert Mueller's probe. Papadopoulos came to the FBI's attention after he allegedly told an Australian diplomat he knew that Russians were in possession of emails belonging to 2016 Democratic presidential nominee Hillary Clinton. (Alex Wong/Getty Images)

Investigators wiretapped Page because of his suspected links with Russians.

The FBI also used an informant, a Cambridge University professor named Stefan Halper, to contact Papadopoulos. The bureau reportedly dispatched an undercover investigator to accompany and pose as Halper's research assistant, using the alias Azra Turk, to meet with Papadopoulos.

Papadopoulos, a former member of Trump's foreign policy panel, came to the attention of investigators after he allegedly boasted in 2016 to an Australian diplomat that he knew Russian hackers had thousands of emails belonging to Democratic presidential nominee Hillary Clinton. Papadopoulos said he offered the Trump campaign access to his contacts to reach out to Russia.

He later pleaded guilty to lying to federal officials about his contacts with Russians and served 12 days in prison.

To initiate its counterintelligence inquiry and conduct electronic surveillance on Page, the FBI secured a "FISA warrant" in October 2016.

What's a FISA warrant?

U.S. Attorney General William Barr testifies before a Senate Judiciary Committee hearing on the Justice Department's investigation of Russian interference with the 2016 presidential election on Capitol Hill in Washington on May 1. (Clodagh Kilcoyne/Reuters)

The acronym refers to the Foreign Intelligence Surveillance Act.

FISA warrants are issued by a FISC, or Foreign Intelligence Surveillance Court, which reviews requests for warrants for wiretapping when the court agrees there is some threat to U.S. national security that justifies the surveillance.

The court, composed of judges from district courts and courts of appeals, conducts its business in secret owing to the need for sensitive intelligence-gathering.

A FISA warrant does not allow for wiretapping in entirely domestic conversations, but if a call involves a foreign power or "agent of a foreign power," the intelligence can be collected.

Was the surveillance illegal?

There's no evidence of that, despite Trump's claims.

The use of confidential informants or undercover sources in FBI operations is lawful, as is the use of court-approved warrants to intercept electronic communications.

"The difference is 'lawful versus unlawful'; it's the same thing as the difference between a search and a burglary," said Litman, the former U.S. deputy assistant attorney general.

"Spying is something that people do illegally, usually on people from another country. Surveillance is something that follows very strict rules ... it does appear that this was lawful surveillance."

Litman said Trump's use of the term "spying" seems designed to draw an emotional response and the suggestion of impropriety.

If it's not spying, what was it?

Pressed about whether the bureau's actions amounted to "spying," FBI director Christopher Wray rejected that description.

"That's not the term I would use," he told a congressional hearing about the FBI's budget this month.

FBI head Christopher Wray was nominated by Donald Trump after the dismissal of James Comey, but he earned criticism from the president after recent testimony he gave on Capitol Hill. (REUTERS)

Simply put, nobody in the intelligence community refers to their work as "spying," said former top FBI official Greg Brower, who led the bureau's Office of Congressional Affairs.

"To use the term 'spying' with respect to what the FBI does, in terms of court-approved counter-intelligence surveillance reflects on the lack of even the most fundamental understanding of how the FBI and our intelligence community works," Brower said.

"We're talking about a very regimented process, involving multiple layers of review within the FBI and within the leadership of the Justice Department, and ultimately approved by the court."

For that reason, he said, Justice Department and intelligence officials were likely baffled by Barr invoking the word. Investigators don't speak in those terms, referring instead to surveillance.

"'Spying' is not the appropriate term, it's not how it's described in the intelligence community," Brower said. "It connotes a whole different sort of activity that isn't something the FBI does. It's just not accurate."