James Clapper, former Director of National Intelligence, told CNN that Comey's sacking has led to concern in the bureau tasked with proving alleged Russian meddling in the November presidential election that brought Trump to the White House.
"I spoke to one (of the FBI employees) last night at a dinner who was quite upset about it. And I think that reflects the feeling, the widespread feeling, in the FBI," he said.
"I will just say that the developments of the past week are very bothersome, very disturbing to me," Clapper said.
Last week, Trump fired Comey, who was leading an investigation into the alleged Russian interference.
Comey's sudden removal from the post set off a political firestorm in Washington, fuelling fears that Trump was trying to stifle the bureau's probe into alleged Russian meddling.
"People had issues with Director Comey's - some of his decisions. That's fine. People took issue with decisions I made. That's part of the deal. But, I think, as far as his stature as a leader and his integrity, people are very upset about the way he was treated," Clapper said.
"I'm pretty familiar with the bureau and its people. It's a national treasure. It's very disturbing to me that the negative morale impact this event has had," he said.
He also said the "assault" on American institutions was not limited to Trump. He said the alleged Russian meddling in the US election system also compromised the institutions.
"In many ways, our institutions are under assault, both externally - and the big news here is the Russian interference in our election system. And, I think, as well our institutions are under assault internally," Clapper said.
Asked if by "internally" he meant Trump, Clapper said: "Exactly".