Donald Trump faced a wave of criticism after bringing up a notorious conspiracy theory that linked Morning Joe host Joe Scarborough to the death of a young female staffer 16 years ago.

Scarborough, a former congressman, has been the source of internet conspiracies after Lori Klausutis was found dead in his Florida district office on 19 July 2001.

Despite Scarborough being out of town at the time of the incident and a medical examiner ruling that Klausutis had died after falling and hitting her head on a desk, conspiracy theorists have claimed that he was somehow behind it. Now, the president has lended credence to the speculations in a tweet against NBC News.

"So now that Matt Lauer is gone when will the Fake News practitioners at NBC be terminating the contract of [MSNBC president] Phil Griffin? And will they terminate low ratings Joe Scarborough based on the 'unsolved mystery' that took place in Florida years ago? Investigate!" Trump tweeted.

Scarborough responded to the tweet by claiming the president "is not well".

"Looks like I picked a good day to stop responding to Trump's bizarre tweets. He is not well," the morning show host wrote.

Others also jumped to his defence, including his brother, George Scarborough, a noted Trump supporter. "Outrageous reference re @JoeNBC where @potus @realDonaldTrump just crossed the line with me. I need to see a public apology to my brother Joe or else he has lost my support for good...which is saying a ton given all the support I've shown him in past 2 years," George wrote.

CNN anchor Jake Tapper called out Trump's attack of the MSNBC host as "indecent," The Hill reported.

"This is the president attempting to exploit the tragic death of a young woman—one who had heart problems and hit her head when she fell—to score a cheap spurious political point. Indecent. Inhumane," Tapper tweeted.

Scarborough, who was never a suspect in the investigation, has warded off critics before who have hoped to use Klausutis' death against him. A medical examiner reported that an "undiagnosed heart condition" contributed to Klausutis' death, which was ruled "an accident".