Republican presidential frontrunner Donald Trump suggested Sunday that he could have prevented the 9/11 attacks had he been president in 2001 -- escalating his feud with primary rival Jeb Bush about the fatal terror strikes.

Trump, a first-time candidate, implied his stance on immigration could have kept out the terrorists who slipped into the United States and trained in the country to hijack the four commercial airliners and kill nearly 3,000 people on American soil on Sept. 11, 2001.

“I am extremely, extremely tough on illegal immigration,” Trump told "Fox News Sunday." “I believe that if I were running things,  … I doubt that those people would have been in the country.”

The 19 hijackers crashed one airliner each into the Pentagon and the twin World Trade Center towers in New York City, roughly nine months after Bush’s older brother, President George W. Bush, took office in 2001.

Passengers in one airliner overpowered the radical Islamic hijackers, forcing the craft to crash in Shanksville, Pa., with no survivors.

The Trump-Bush feud essentially started during the second 2016 GOP presidential primary debate when Jeb Bush defended his brother against Trump’s criticism about the attacks.

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“You remember the rubble at the World Trade Center? He sent a clear signal that the United States would be strong and fight Islamic terrorism, and he did keep us safe,” Bush said to huge audience applause.

Trump responded: “You feel safe right now? I don’t feel so safe.”

Bush, a former Florida governor, has since shaped his response to suggest his brother united Americans and kept then safer after the attacks.

The exchanges also have put Bush in a challenging position, defending his family while trying to distance himself from Bush political dynasty, included shortcomings in the administrations of his brother and father, George H.W. Bush.

On Friday, Trump returned to his attacks when talking on Bloomberg TV about how and why, if elected, he could best handle national emergencies.

“Blame him or don’t blame him, but (Bush) was president,” said the billionaire New York real estate mogul. “The World Trade Center came down during his reign.”

Hours later Jeb Bush tweeted, “How pathetic for @realdonaldtrump to criticize the president for 9/11. We were attacked & my brother kept us safe.”

Trump also said Sunday that during the debate he was just responding to Bush saying the country was safe under his brother’s watch.

“I'm not blaming anybody,” Trump said. “But the World Trade Center came down.  So when he said, we were safe, that's not safe.  … It was probably the greatest catastrophe ever in this country.”