Joe Scarborough's brother — a supporter of President Trump — said Trump "crossed the line" with his tweet appearing to call for an investigation into a dead staffer found in the former GOP congressman's office in 2001.

"Outrageous reference re @ JoeNBC where @ potus @ realDonaldTrump just crossed the line with me," George Scarborough tweeted Wednesday morning.

"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."

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. https://t.co/3lXfBPnaTV — George Scarborough (@GeoScarborough) November 29, 2017

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George Scarborough wrote that Trump mentioning fake news regarding his brother gives him "serious pause regarding" Trump's "'news' sources."

He tweeted that if Trump doesn't apologize, the president will lose his "public support & my vote in the next election."

"No [one] is deserving of lies being told about them in the public domain where idiots believe everything they see on social media," he tweeted.

"For all the Haters blasting me for defending my brother ... still [Trump] went too far w/me w/tweet. Done," he also wrote.

For all the Haters blasting me for defending my brother from @realDonaldTrump now, I KNOW this is #FakeNews, & apologies for being an R & liking SCOTUS pick, booming stock mkt , 3%+ GDP growth, low unemployment, & other policies, still @potus went too far w/me w/tweet . Done. — George Scarborough (@GeoScarborough) November 29, 2017

Trump on Wednesday mentioned an "unsolved mystery" that took place in Florida years ago in a tweet questioning when Joe Scarborough and MSNBC president Phil Griffin will be fired.

"So now that Matt Lauer is gone when will the Fake News practitioners at NBC be terminating the contract of Phil Griffin?" Trump tweeted.

"And will they terminate low ratings Joe Scarborough based on the 'unsolved mystery' that took place in Florida years ago? Investigate!"

The president was referring to Lori Klausutis, a 28-year-old office staffer who worked for Scarborough while he was in Congress and was found dead in his district office in July 2001.

A local medical examiner ruled the death an accident, saying Klausutis had collapsed and hit her head on the side of a desk. She had an undiagnosed heart condition, according to the medical examiner, and the head trauma caused a fatal blood clot.

Joe Scarborough, who responded to Trump's tweet by saying that Trump is "not well," is a frequent critic of the president. Earlier this year, he announced he was leaving the Republican Party.

George Scarborough, however, has supported President Trump. During a 2016 campaign rally, George Scarborough spoke and endorsed Trump.