Trump insisted he isn't "blaming anybody," but repeatedly reminded Fox News viewers that the "worst attack in the history of our country" occurred on Bush's watch and suggested that the attacks could have been prevented.
"The fact is we had the worst attack in the history of our country during his reign. Jeb (Bush) said we were safe during his reign. That wasn't true," Trump said. "And I'm not blaming anybody and I'm not blaming George Bush, although if you look at his three primary agencies, they hated each other, they weren't talking ... And a good leader would've made sure that they would get along and talk and lots of other things happen."
Trump was referencing how in the run-up to the 9/11 attacks top law enforcement and intelligence agencies including the CIA and the FBI weren't coordinating as closely as they do now.
Former Florida Gov. Jeb Bush, one of Trump's rivals for the 2016 GOP presidential nomination, said during the last Republican debate that his brother, the former president, "kept us safe." Trump on Friday reopened that feud and challenged that assertion.
Jeb Bush responded swiftly
after Trump's initial comments, calling the line "pathetic."
"How pathetic for @realdonaldtrump to criticize the president for 9/11. We were attacked & my brother kept us safe," he tweeted.
Trump even suggested Monday that the Bush administration "knew in advance" the U.S. would be attacked.
"CIA Director George Tenet knew in advance that there was going to be an attack," Trump said."He knew in advance that there was going to be an attack."
While Tenet did go to the White House two months before 9/11 with intelligence reports suggesting al-Qaeda was plotting a terrorist attack on the U.S., Tenet did not know when or how those attacks would unfold and he did not know how reliable the intercepted chatter was.
Former Trump political aide Roger Stone, who is also a well-known conspiracy theorist, pushed a similar line hours before Trump called into Fox News, tweeting that Tenet "admitted he had 60 day advance warning of attack on America-and did nothing."
Stone also tweeted two days before that ".@realDonaldTrump is right - Bush Admin knew of attack on America 60 days in advance- did nothing."
Trump also said Monday that tougher immigration policies like the ones he would implement if he became president would have thwarted the 9/11 terrorist attacks. Those hijackers entered the U.S. legally, but Trump said he would have implemented tougher visa standards to prevent their entry into the country in the first place. It's unclear what those standards would have been.
Trump also said he would have had a "massive whistleblower system" to gain advance knowledge of the attacks. He did not explain how such a system would work.
The latest sparring between Trump and Jeb Bush is just the latest in a series of feuds that have marked the two contenders' relationship through the primary.
Trump has repeatedly knocked Bush over his last name -- suggesting there should be no more Bushes and Clintons in the White House -- and claimed that the former Florida governor is too "low-energy" to take on the job of commander-in-chief.
After first resisting to engage the brash billionaire, Bush has in recent months taken Trump on directly and forcefully, this weekend explaining on CNN's "State of the Union" that he has "grave doubts" about Trump's preparedness to handle the responsibilities of president of the United States. Bush also linked Trump's candidacy to the antics of Trump's reality show, "The Apprentice."
But Trump's criticism of the two Bushes is doing more than just dragging the former Florida governor and one-time establishment favorite for the nomination into a mud fight. It's also provoking Jeb Bush into repeated impassioned defenses of his brother's tenure as president, which remains contentious.
Jeb Bush struggled early in the campaign to call his brother's decision to invade Iraq a "mistake" -- a judgment that today has broad consensus -- as he almost reflexively defended his brother.
And Jeb Bush did it again this weekend, in the face of Trump's latest attack over 9/11:
"My brother responded to a crisis, and he did it as you would hope a president would do. He united the country, he organized our country and he kept us safe. And there's no denying that. The great majority of Americans believe that," Bush said.