President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE on Thursday attacked James Clapper on Twitter, two days after the former director of national intelligence questioned his fitness for office.

“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 wrote on Twitter.

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? — Donald J. Trump (@realDonaldTrump) August 24, 2017

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Clapper questioned Trump’s fitness for office after the president's remarks at a Tuesday night rally in Phoenix, during which Trump defended his response to violence at a white supremacist rally in Charlottesville, Va.

"I really question ... his fitness to be in this office," Clapper told CNN. "I also am beginning to wonder about his motivation for it, maybe he is looking for a way out."

Trump’s accusation that Clapper lied to Congress is a reference to testimony Clapper provided several years ago about the National Security Agency’s surveillance program.

Clapper in 2013 told Sen. Ron Wyden Ronald (Ron) Lee WydenGOP set to release controversial Biden report Democrats fear Russia interference could spoil bid to retake Senate GOP senator blocks Schumer resolution aimed at Biden probe as tensions run high MORE (D-Ore.) that the United States does not collect data on Americans. Months later, Clapper apologized, saying he was mistaken in his testimony and that his response to Wyden’s question was “clearly erroneous.”

It's not clear, however, what letter Trump was referring to in his tweet.