Frank Figliuzzi, a national security contributor for NBC News, joined MSNBC's "AM Joy" on Saturday.
Host Joy Reid noted the "extraordinary" reporting from The New York Times that the FBI opened a counterintelligence investigation into Trump in 2017.
"I think this is particularly sobering, even for career counterintelligence professionals who always in the back of their minds think there is an outside possibility someone could rise to high office who might be playing for another team," Figliuzzi explained.
"To see this in writing, to hear this report -- if it's accurate to say the bureau actually opened a case on Donald J. Trump -- is really like hitting the American people in the gut," he continued.
Figluizzi predicted the FBI had additional evidence.
"It means likely, Joy, even though the article is citing the public behavior of the president, as you just noted, in order to get this to pass muster, in order to get this through FBI headquarters, through teams of lawyers, across the street to the Department of Justice, to be the title name of an investigation, I am virtually certain that more evidence than just the public behavior of the president is involved in this and I think that evidence is likely classified evidence," he explained.
Reid noted that the National Security Agency had recently relaxed rules on sharing information with other governmental departments.
"Does that tell you -- as a former assistant director for counterintelligence at the FBI -- that the FBI might have intercepted phone communications and signal intelligence about ongoing cooperation between the president of the United States and Russia?" Reid asked.
"The short answer is yes," Figliuzzi replied. "But I don't even think that those relaxed regulations may play into that or not. They might, but from day one, Joy, you've heard the intelligence professionals saying there is much more to this iceberg than just the tip."
"That is what we call the dark side -- the signals intelligence, the intercepts worldwide," he continued.
Signals intelligence refers to electronic communications that have been intercepted by intelligence agencies.
"We know from reporting throughout this case that allied partners have shared intelligence, whether it's Australians or Brits or others," Figliuzzi noted.
"This is really the hard part to get your arms around, literally our allies may hav been helping -- and our intelligence community may have been supporting -- a case and evidence development against our own president," he concluded.
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