WASHINGTON (AP) — Former FBI Director James Comey asserted Thursday that President Donald Trump fired him to interfere with his investigation of Russia's role in the 2016 election and its ties to the Trump campaign.

"It's my judgment that I was fired because of the Russia investigation," Comey told the Senate intelligence committee in explosive testimony that threatened to undermine Trump's presidency.

"I was fired in some way to change, or the endeavor was to change, the way the Russia investigation was being conducted," Comey testified under oath. "That is a very big deal, and not just because it involves me."

Comey also accused the Trump administration of spreading "lies, plain and simple" about him and the FBI in the aftermath of his abrupt firing last month, declaring that the administration then "chose to defame me and, more importantly, the FBI" by claiming the bureau was in disorder under his leadership. And in testimony that exposed deep distrust between the president and the veteran lawman, Comey described intense discomfort about their one-on-one conversations, saying he decided he immediately needed to document the discussions in memos.

"I was honestly concerned that he might lie about the nature of our meeting, so I thought it really important to document," Comey said. "I knew there might come a day when I might need a record of what happened not only to defend myself but to protect the FBI."