Donald Trump warned Republican rival Chris Christie Monday to "be careful" when criticizing him, hours after the New Jersey governor flat out rejected Trump's claim that Muslims in Jersey City, N.J., celebrated the Sept. 11 terror attacks.

"Definitively, I've said this a number of times: It didn't happen," Christie told reporters in N.H. Monday. "The fact is, people can say whatever they like. But the facts are the facts, and that did not happen in New Jersey that day, and it hasn't happened since."

When asked last week if residents of his state had celebrated the downing of the Twin Towers, Christie told reporters, "I don't remember that, no."

"He was very weak the other day," Trump said Monday, referring to Christie's initial refusal to identify his opponent's claim as true or false.

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