NEW YORK (CBSNewYork)Donald Trump is not backing off from his criticism of former President George W. Bush over 9/11 — and presidential candidate Jeb Bush is blasting back.

The latest fight between Trump and Jeb Bush began Friday when Trump said, “When you talk about George Bush, I mean, say what you want, the World Trade Center came down during his time.”

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Two days later, Trump, appearing on “Fox News Sunday” doubled down: “Now, am I trying to blame him? I’m not blaming anybody. But the World Trade Center came down, so when he said we were safe, that’s not safe. We lost 3,000 people.”

Jeb Bush, George W. Bush’s brother, responded by releasing a campaign video, further dissing Trump, saying his continuous attacks expose him as a reality TV host.

“It doesn’t show that he’s a serious person as it relates to him being commander-in-chief,” the former Florida governor said. “Across the spectrum of foreign policy, Mr. Trump talks about things as though he’s still on ‘The Apprentice.'”

The two sides have also traded barbs on Twitter.

But Trump insists things would have been different on 9/11 if he had been president.

“I am extremely, extremely tough on immigration. I am extremely tough people coming into this country,” Trump said. “I believe if I were running this country … I doubt that those people would have been in this country.”

All 19 Sept. 11 hijackers entered the country on temporary visas — one for a student, 14 under tourist status and four as business visas. At least six violated those visas.

Other Republican candidates are questioning Trump’s criticisms.

“I would probably ask him what he meant by that,” Ben Carson said. “I seriously doubt that he’s saying that George W. Bush is to blame for it.”

On Monday afternoon, Trump tweeted out a link to a three-year-old New York Times story about the classified presidential daily brief that President Bush received about a month before 9/11. The headline of the brief was “Bin Laden Determined to Strike in the U.S.”