Former Director of National Intelligence James Clapper questioned Donald Trump’s fitness for office, so the President fired back with an angry tweet Thursday morning.

Clapper drew headlines Wednesday after questioning Trump’s mental state following a 77-minute rally in Phoenix, Arizona, where the president threatened a government shutdown, criticized Republican senators and argued that the media is unpatriotic.

“I don’t know when I’ve listened and watched something like this from a President that I’ve found more disturbing. Having some understanding of the levers of power that are available to a President if he chooses to exercise them, I found this downright scary and disturbing,” Clapper told CNN’s Don Lemon Wednesday evening.

“Are you questioning his fitness?” Lemon asked.

“Yes I do,” Clapper said. “I really question his ability — his fitness to be in this office — and I also am beginning to wonder about his motivation for that.”

The following day, Trump fired back, noting the controversy over Clapper’s testimony to Congress that the U.S. was not collecting data on millions of Americans, which was later disproved by documents leaked by Edward Snowden. Clapper has said he misspoke.

“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?” Trump tweeted at 9:15 a.m., the sixth of seven Twitter posts he wrote or shared that morning.

Shawn Turner, Clapper’s spokesman, said the letter Trump was referring to was the note he had prepared for the winner of the Presidential election, that was handed to to Trump with the presidential briefing the morning after he had won. Clapper had prepared nearly identical notes for Trump and Clinton, Turner said. Both contained congratulatory sentiments and reiterated the values and commitment of the intelligence community to holding truth to power.

Turner said Clapper, who was unavailable to speak with TIME, had no comment on the President’s tweets this morning.

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Write to Alana Abramson at Alana.Abramson@time.com.