The White House is blasting as “absurd” a blockbuster new report that the FBI opened an investigation into the whether the president of the United States was working on behalf of the Kremlin. But respected former FBI special agents tell The Daily Beast such a momentous step would not be taken without “serious and substantial evidence.”

They told The Daily Beast that the senior-most levels of the FBI and Justice Department would have known about an event they considered without precedent in bureau history.

“This is uncharted territory,” said Ali Soufan, a retired FBI counterterrorism special agent. “I don’t believe that it had happened before… Ever.”

On Friday night, The New York Times reported that FBI agents opened a counterintelligence investigation in May of 2017 into whether President Trump had been operating “on behalf of Russia against American interests.”

If the Times is correct, then the FBI overcame its reluctance to investigate Trump after he fired one of its own: former director James Comey, whom the president dismissed that same month. One implication of that timeline is that Robert Mueller, who was appointed as special counsel a week later, would have had access to their investigation for at least substantial portions of his own.

Sen. Richard Blumenthal, a Democrat on the committee that oversees the FBI, said the Times’ reporting was intriguing, and “another powerful piece of the mosaic” on Trump’s relationship with Russia.

The Times’ revelations also may reignite Republican criticism of FBI officials who privately expressed concern about Trump while working on investigations concerning him. After text messages between former FBI lawyer Lisa Page and former counterintelligence agent Peter Strzok became public—revealing their criticism of Trump—congressional Republicans brought them in for grilling sessions. Page in particular faced protracted and ugly attacks from Fox News opinion hosts. And the president himself also tweeted numerous times about her, sarcastically calling her “the lovely Lisa Page.” But the Times’ reporting indicates they were far from alone in their concerns about Trump’s posture towards Russia.

“There are a variety of ways to gather information about foreign efforts to influence a U.S. official that don’t require the sensitive step of targeting that official’s communications, and those who are criticizing the FBI for pursuing a counterintelligence investigation are doing so without any knowledge of what investigative steps were actually undertaken,” a Justice Department trial attorney told The Daily Beast.

Mike German, another retired FBI special agent, said an investigation like this would likely have required assent from the very top of the Justice Department.

“It would be most likely that the highest levels of of the FBI and DOJ signed off on the investigation,” German said.

The FBI issues specific guidelines on how agents should handle different levels of counterintelligence investigations. A preliminary investigation requires a low standard of evidence, that “information or an allegation” exists as to a crime or security threat. A full investigation requires “specific and articulable facts” raising a “reasonable indication” of the same. Both are well short of the probable cause needed for an indictment. It is unclear which sort of investigation, if any, Trump was or is under.

“Of course, with the U.S. president as a subject, the threshold would be much higher than normal,” German said.

The Justice Department trial attorney agreed, saying, “foreign efforts to influence presidential candidates are of course supremely serious.”

Soufan added that to open a case “on any official, or high-ranking official, requires some serious deliberations.” He continued: “Imagine if it is a case on the president of the U.S. acting as an agent of a hostile foreign power.”

Soufan went on to note that to open up a counterintelligence investigation into Trump would require “serious and substantial” levels of evidence of a relationship with a hostile foreign power.

It’s unclear whether or not that investigation is over. Former Justice Department officials told The Daily Beast that counterintelligence probes, especially those into a president, would take years to complete.

"They take a long time. They're not over quickly. And based on the president’s public statements and actions, I think you have to open a cointel investigation,” said one former senior DOJ official with knowledge of such counterintelligence investigations. “You might never know that it's resolved. These cases often never see the inside of a courtroom. The findings are often kept within the intelligence community, indexed and filed away.”

The FBI did not comment for this story. The Department of Justice declined to comment as did the Special Counsel’s Office.

The White House was quick to lash out, however. “Unlike President Obama, who let Russia and other foreign adversaries push America around, President Trump has actually been tough on Russia,” White House Press Secretary Sarah Sanders said in a statement, adding that the Times report was “absurd.”

Trump’s attorney in the Mueller probe, former New York mayor and federal prosecutor Rudy Giuliani, dismissed the Times report and called for an investigation into Trump’s FBI investigators.

“The investigation goes back a year and a half, if it ever really happened. If it did after a year and a half if there has been no information forthcoming it obviously came to the same result as the first Peter Strzok inspired counter-intelligence investigation,” Giuliani told The Daily Beast. “As Strzok and Comey said it found no evidence of any inappropriate conduct on the part of the President. In any event, it was, if it happened, yet another example of the extent to which Trump haters like Strzok and Comey would go to carry out their insurance policy to overthrow the Trump Presidency. It calls again for a full and vigorous investigation of the so-called investigators.”

But just the idea of the sitting U.S. president being probed as a possible agent of a foreign nation, especially one like Russia, has former senior government officials and former prosecutors grasping for the right words to describe the news.

“It’s mind-blowing and even though it’s separate from criminal inquiry, any determination that the president of the U.S. had been or was a Russian agent would be out of The Manchurian Candidate,” said Harry Litman, a former U.S. attorney. “And I would have to imagine it would make it untenable for him to be president but it would also be the political scandal of all time.”