Andrew Napolitano, a retired judge, said former national security adviser Michael Flynn’s guilty plea was a “nightmare” for President Trump.

“This is probably the tip of a prosecutorial iceberg, so to speak,” Napolitano, a Fox News contributor, said on the network’s “Outnumbered” on Friday. “It’s a nightmare for Donald Trump because Gen. Flynn was his constant companion on foreign policy and national security matters from June of 2015 to the time the president fired him.”

Flynn pleaded guilty on Friday in a federal court in Washington for lying to the FBI in January about his contacts with the Russian ambassador. Flynn was charged on one count of “willfully and knowingly” making “false, fictitious and fraudulent statements” to the FBI about communications exchanged with then-Russian Ambassador Sergey Kislyak shortly after Trump took office. Flynn said he is cooperating with special counsel Robert Mueller.

ABC News reported that Flynn intends to testify against Trump and members of his family and plans to say then-president-elect Trump told him to make contact with Russians.

Flynn was removed from his post at the White House earlier this year after it was revealed he misled the administration about his sanctions-related communications with the Russians.

Editor's note: This story has been updated to reflect ABC News' acknowledgment that when Michael Flynn reportedly plans to testify that President Trump told him to make contact with the Russians, he will say it happened when Trump was president-elect, not a candidate.