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President Donald Trump appeared to threaten James Clapper, former director of national intelligence, on Thursday with a tweet suggesting he once received a flattering letter written by Clapper, who had earlier this week questioned Trump’s 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? — Donald J. Trump (@realDonaldTrump) August 24, 2017

The attack on Clapper’s credibility follows his appearance on CNN, where he condemned Trump’s volatile rally in Phoenix, Arizona, on Monday. The former intelligence chief also expressed concern over Trump’s control over the country’s nuclear codes—access Clapper described as “pretty damn scary.”

Trump has used Twitter to fire off similar threats against his opponents—instances that have further complicated the president’s legal troubles. In May, shortly after he fired James Comey as FBI director, Trump implied he possessed secret recordings of their conversations inside the White House. Prior to that, Trump made an explosive—and false—claim that during the 2016 election, Barack Obama had illegally ordered a wiretap of his campaign at Trump Tower.

The president has refused to correct or apologize for either claim.