He then laid out what he would do to address the attack as president, assuming that it was an act of terror.

"I would be making it very, very hard for people to come into our country, for one thing, from terrorist areas. I would be so extreme in terms of documentation," he said.

He said that Hillary Clinton wants to resettle more Syrian refugees than President Obama.

"Obama is allowing a lot of people to come in. We have no idea who they are. They're from Syria, maybe, but they have no paperwork many times. They don't have proper documentation. I would make it – I would not allow people to come in from terrorist nations. I would do extreme vetting," he said.

Van Susteren noted that the recent shooting in Orlando was carried out by an American. In response, Trump said that "second generation turns out to be very bad for whatever reason."

The presumptive Republican presidential nominee later called into Bill O'Reilly's Fox News show, where O'Reilly said that there is a "world war scenario." Trump agreed.

And when O'Reilly asked Trump if he would ask Congress to declare war on the Islamic State as president, Trump said, "I would. I would. This is war."

At another point in the interview, Trump appeared to suggest that Clinton and Obama share some of the blame for the Islamic State's rise.

"She's very weak. She's actually the one, and him, that really caused much of this problem," he said.

O'Reilly attempted to finish Trump's thought by saying, "By the failure to keep troops in Iraq."