We all remember the days when Republican presidential hopeful Donald Trump insisted on seeing President Obama’s birth certificate, insinuating he was Kenyan-born, and ineligible for office. Trump, once a birther, may now be a 9/11 truther.

In his latest feud, billionaire Donald Trump has traded insults with Republican rival Jeb Bush, and it all began when the former bashed the latter’s big brother.

"I’m not blaming George Bush," Trump said on Fox News Sunday. "But I don’t want Jeb Bush to say, 'My brother kept us safe,' because September 11 was one of the worst days in the history of this country."

— Donald J. Trump (@realDonaldTrump) October 18, 2015

This followed, of course, Trump’s insistence that had he been president in 2001, the World Trade Center would still be standing.

Jeb took to CNN to defend his brother’s honor.

"My brother responded to a crisis, and he did it as you would hope a president would do," he said on State of the Union.

— Jeb Bush (@JebBush) October 16, 2015

True to his style, Trump has refused to apologize for his criticism of the Bush administration, and on Tuesday, the real estate mogul doubled down on his claims.

"Look, his brother gets hit on, he’s a loyal person, he’s loyal to his brother," Trump said, referring to Jeb Bush. “But his brother made some mistakes.

"They did know it was coming," he said referring to the 9/11 terrorist attacks. "George Tenet, the head of the CIA, told them it was coming. So they did have advanced notice."

Curiously, though, even with this lack of foresight, Trump doesn’t blame the Bush administration for the attacks.

"I don’t blame him for that," Trump said of the Bush’s role in the tragedy.

"He was president, okay? Blame him or don’t blame him, but he was president. The World Trade Center came down during his reign."

The third Republican presidential debate will take place next week. Expect another heated exchange between Trump and Bush. Maybe Ben Carson will even jump into the fray.