President Donald Trump revived an unfounded conspiracy theory about the MSNBC host Joe Scarborough on Wednesday.

The conspiracy theory centers on an aide who died in Scarborough’s office in 2001. Authorities have said the death was the result of a heart condition.

President Donald Trump on Wednesday called for the “Morning Joe” host Joe Scarborough – as well as MSNBC’s president, Phil Griffin – to be fired, citing an unfounded conspiracy theory about the 2001 death of an intern in Scarborough’s congressional office.

Trump’s tweet mentioned NBC’s firing of the longtime “Today” show host Matt Lauer after receiving a complaint that Lauer had engaged in sexual misconduct with an NBC employee.

“So now that Matt Lauer is gone when will the Fake News practitioners at NBC be terminating the contract of Phil Griffin? And will they terminate low ratings Joe Scarborough based on the ‘unsolved mystery’ that took place in Florida years ago? Investigate!” Trump wrote.

Trump was referring to a longtime obsession of online conspiracy theorists: the death in 2001 of an aide to Scarborough, a Republican congressman from Florida at the time, named Lori Klausutis.

Authorities at the time said they found no evidence of foul play – a medical examiner said that because of a heart problem, Klausutis lost consciousness and collapsed in Scarborough’s district office, hitting her head.

Scarborough responded quickly on Wednesday in a tweet, repeating his suggestion that Trump is mentally unfit for office.

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Looks like I picked a good day to stop responding to Trump's bizarre tweets. He is not well. https://t.co/XJhW5ZHNfs — Joe Scarborough (@JoeNBC) November 29, 2017

The tweets capped several days of back-and-forth between the two, who were friends before Trump became president.

The “Morning Joe” host has since been highly critical of Trump’s rhetoric, tweets, willingness to engage with conspiracy theories, and policies on issues like immigration. He has even written anti-Trump protest songs.

The New York Times reported on Tuesday that Trump has continued to publicly and privately entertain conspiracy theories, including the long-debunked claim that former President Barack Obama was not born in the US. He also continues to falsely claim that immigrants living in the US without permission helped Hillary Clinton win the popular vote in the 2016 presidential election, the report said.

And a Times report over the weekend said Trump had suggested the 2005 “Access Hollywood” tape in which he boasted about groping women isn’t real, though he publicly acknowledged its authenticity during the campaign.