Fox News host Greta Van Susteren questioned Donald Trump, who has proposed temporarily banning all Muslim immigration into the U.S., Wednesday for specifics on his policies about Muslims who are already living in the country.

In the interview, Van Susteren noted the fact that, should Trump’s plan be implemented, it would not have necessarily prevented the gruesome Orlando terror attack since the radicalized Muslim who carried out the assault was born in the United States.

“We’ve already got so many of those people that you would now seek to ban,” Van Susteren said. “They’re already in the United States. What would you do — you gonna go round them up? What would you do with the people and how do you handle the fact that so many are lawful?”

In response, Trump emphasized his proposal is only temporary and that Muslims living within the U.S. must report suspicious behavior to authorities because, according to the businessman, “it always turns out that people knew.” Trump noted the fact that Orlando killer Omar Mateen’s wife “knew everything.”

“But they don’t report it. They never report it to the authorities so we have these catastrophic events. If they don’t report, we’re never going to figure out what’s going on,” said Trump.