Fired FBI Director James Comey said Thursday the Trump administration lied and "chose to defame" him in explaining his ouster last month.

"Although the law required no reason at all to fire an FBI director, the administration then chose to defame me and more importantly the FBI by saying that the organization was in disarray, that it was poorly led, that the workforce had lost confidence in its leader," Comey said in opening remarks to members of the Senate intelligence committee. "Those were lies, plain and simple."

The former director, testifying about his dealings with President Donald Trump in a packed hearing room in the most anticipated congressional hearing in recent memory, also said he was "confused" and "increasingly concerned" by the shifting and conflicting reasons that Trump and his deputies gave for his firing.

"It confused me when I saw on television the president saying that he actually fired me because of the Russia investigation, and learned again from the media that he was telling privately other parties that my firing had relieved great pressure on the Russia investigation," Comey said.

"I was also confused by the initial explanation that was offered publicly, that I was fired because of the decisions I had made during the election year," he continued, a reference to his July 2016 press conference to discuss the federal investigation into Hillary Clinton's use of a private email server. "That didn’t make sense to me for a whole bunch of reasons, including the time and all the water that had gone under the bridge since those hard decisions that had to be made."

Later, under questioning from Democratic Sen. Mark Warner of Virginia – the committee's vice chairman – Comey said he decided to keep documentation of his interactions with the president following a January meeting due to "a combination of things: I think the circumstances, the subject matter and the person I was interacting with."

Comey said he was alone with the president-elect during that meeting, discussing matters connected to the FBI's core responsibility and Trump himself.