(CNN) When former Director of National Intelligence James Clapper questioned President Donald Trump's "fitness to be in this office" after the President's wild speech in Phoenix Tuesday night, it was likely only a matter of time before the President aimed his Twitter ire at the retired general.

Indeed, Thursday morning came the tweet from the President: "James Clapper, who famously got caught lying to Congress, is now an authority on Donald Trump. Will he show you his beautiful letter to me?"

In an interview, Clapper told CNN about the "beautiful letter."

"The night before the election, we deployed two teams so that we would be ready to provide a PDB brief the next morning to whomever won," Clapper recounted, referring to the Presidential Daily Briefing. "I hand-wrote almost identical short notes to each of the two candidates to accompany the first brief as President-elect; only one actually got deployed -- the one to him."

The note to Clinton, which went undelivered, congratulated her on her victory and said the intelligence community stood by to serve her with the best intelligence it could muster. The note to Trump said the same thing.

"I went on to say that I hoped he would abide by the long-standing principle of the IC always telling 'truth to power,'" Clapper told CNN, noting that he was paraphrasing since he doesn't have a copy of the note. "IC" is shorthand for intelligence community.

On January 6, leaders of the intelligence community traveled to Trump Tower in New York City to brief then-President-elect Trump on the Intelligence Community Assessment on Russian interference in the election. Clapper remembers Trump specifically thanking him for the note three times.

"Of course he shot the messengers on the 11th of January when he characterized us as Nazis for having delivered truth to power," Clapper added, referring to President Trump's tweet protesting CNN's report on how intelligence leaders had selected then-FBI Director James Comey during that trip to brief him on the existence of a dossier put together by a former British intelligence agent. The dossier included reporting on various claims Russians were making about damning information they allegedly had on him. "Intelligence agencies should never have allowed this fake news to "leak" into the public," he tweeted. "One last shot at me. Are we living in Nazi Germany?"

Clapper also noted to CNN that he does not consider himself an "authority" on President Trump, as the president tweeted. "My comments are based on what I've observed him say and do -- just like everyone else," he told CNN.