Story highlights Former FBI Director James Comey is scheduled to testify Thursday before the Senate

Comey previewed his testimony as well as conversation he had with President Donald Trump

(CNN) Testimony released from former FBI Director James Comey on Wednesday helped shed light on a claim by President Donald Trump that Comey had thrice told him that the President himself was not under investigation.

Trump said repeatedly last month that Comey assured him on three separate occasions that the President was not under Justice Department investigation over whether his campaign colluded with Russia.

"While I greatly appreciate you informing me, on three separate occasions, that I am not under investigation, I nevertheless concur with the judgment of the Department of Justice that you are not able to effectively lead the Bureau," Trump wrote in his letter firing Comey.

The first time, on January 6, occurred when Comey went to Trump Tower to brief the President-elect along with intelligence leaders. As CNN reported, it was agreed that Comey would pull aside Trump and explain a dossier of allegations of Russian ties to Trump's campaign . Comey had discussed with FBI leadership what to say if Trump asked if he was under investigation. Comey writes he was ready to assure Trump he was not. In the end: "During our one-on-one meeting at Trump Tower, based on (Trump's) reaction to the briefing and without him directly asking the question, I offered that assurance."

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