Donald Trump said Sunday that he believes refugees will launch a terrorist attack against the United States comparable in size to the Sept. 11, 2001 attacks.

Asked on the National Border Patrol Council's Green Line radio show if he thought it would take another terrorist attack the size of the Sept. 11 attacks to make people "wake up about border security," Trump answered, "I do, I actually do."

"Bad things will happen – a lot of bad things will happen," continued Trump. "There will be attacks that you wouldn't believe. There will be attacks by the people that are right now that are coming into our country, because, I have no doubt in my mind."

Trump said refugees coming into the U.S. had cellphones with ISIS flags on them, and questioned how refugees could afford cell phones, suggesting ISIS paid the monthly fees.

"I mean you look at it, they have cell phones," said Trump. "So they don't have money, they don't have anything. They have cell phones. Who pays their monthly charges, right? They have cell phones with the flags, the ISIS flags on them. And then we're supposed to say, 'isn't this wonderful that we're taking them in?' We're led by people that are either incompetent or they don't have the best interest of our country at heart."