CHICAGO (CBS) — A constitutional law professor from Chicago said former FBI Director James Comey’s testimony before the Senate Intelligence Committee on Thursday is a big step forward in a potential obstruction of justice case against President Donald Trump.

Comey accused the Trump administration of spreading “lies, plain and simple” and “defamed” him and the FBI after he was fired.

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While Comey said he can’t know for sure why he was fired, he noted Trump himself has said he was thinking about the agency’s investigation of Russia’s interference with the 2016 election.

“I take the president at his word that I was fired because of the Russia investigation,” he said.

Comey also recalled Trump asking him to end the investigation of former National Security Adviser Mike Flynn during a private meeting in the Oval Office, after the president had asked Attorney General Jeff Sessions and other advisers to leave. The meeting happened a day after Trump fired Flynn, and Trump said Flynn had done nothing wrong in his conversations with Russian officials.

“He then said, ‘I hope you can see your way clear to letting this go, to letting Flynn go. He is a good guy. I hope you can let this go,'” Comey recounted.

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Comey said he took that “as a direction” to drop the investigation of Flynn’s conversations with Russia,

Loyola University law professor Alexander Tsesis took a break from watching Comey’s testimony to share his thoughts.

“It’s shocking. The degree to which the chief executive, that is to say the president of the United States, was willing to spend time on his own in a deliberate manner, that seemed to pressure the FBI director – without anyone else being present – to drop an investigation that dealt with enormous harms to the U.S. democracy, and our election system, are bombshells,” he said.

Tsesis said he sees Comey’s testimony as a big leap in a potential obstruction case against the president.

“You know, when you consider George Washington, the first president of the United States, who made the model for presidents, to have a president of the United States who is trying to get friends favors and have his family in the highest reaches of secrecy is, in my opinion, very questionable behavior,” he said.

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Trump’s outside counsel was expected to address Comey’s testimony later Thursday afternoon.