Republican presidential candidate Donald Trump gives a thumbs up as he arrives for a campaign rally, Saturday, June 11, 2016, at a private hanger at Greater Pittsburgh International Airport in Moon, Pa. (AP Photo/Keith Srakocic)

WASHINGTON (AP) - Republican Donald Trump asserted Monday there are thousands of people living in the United States "sick with hate" and capable of carrying out the sort of massacre that killed at least 49 people in a Florida nightclub.

"We can't let people in. ... We have to be very, very strong," the presumptive Republican presidential nominee said in one of a host of broadcast interviews ahead of a speech he planned later Monday in New Hampshire.

"The problem is we have thousands of people right now in our country. You have people that were born in this country" who are susceptible to becoming "radicalized," the billionaire real estate mogul told Fox News Channel's "Fox & Friends. He claimed that there are Muslims living here who "know who they are" and said it was time to "turn them in."

The gunman, identified by police as Omar Mateen, a 29-year-old U.S. citizen from Fort Pierce, Florida, opened fire with an assault-style rifle inside a crowded gay nightclub in Orlando early Sunday, killing at least 49 people before dying in a gunfight with police. Another 53 people were hospitalized, most in critical condition.

Trump's longstanding proposal to temporarily ban foreign-born Muslims from entering the United States has triggered outrage from Democrats and Republicans alike, who see it unconstitutional, un-American and counterproductive. But it has helped him win over many primary voters who fear the rise of Islamic extremism and believe that "political correctness" - the fear of offending Muslims - is damaging national security.

Trump said Monday "there are people out there with worse intentions" than the perpetrator of the shootings in Orlando early Sunday. "They have to report these people," he said.

"This is a case of surveillance," he said on CNN. "You will find that many people that knew him (the Orlando shooter) felt that he was a whack job."

"You look at the people that have come to the country, and are here, and for that we need intelligence-gathering," he said. "We have to look at the mosques. The (Muslim) communities know the people that have the potential for blowup."

Trump planned later Monday to further address the deadliest shooting in modern U.S. history in a campaign speech originally intended to attack Hillary Clinton, the presumptive Democratic nominee. That switch came a day after Trump called for Clinton to drop out of the race for president if she didn't use the words "radical Islam" to describe the Florida nightclub massacre.

Trump will retool his talk in New Hampshire to "further address this terrorist attack, immigration and national security," his campaign said Sunday.

A gunman wielding an assault-style rifle and handgun opened fire inside a crowded gay nightclub in Orlando early Sunday, killing at least 50 people before dying in a gunfight with police. Another 53 people were hospitalized, most in critical condition. Authorities identified the shooter as Omar Mateen, a 29-year-old U.S. citizen from Fort Pierce, Florida.

Trump's hardline approach to fighting Islamic terrorism was a hallmark of his primary campaign. Besides proposing a temporary prohibition on foreign Muslims from entering the country, he has and advocated using waterboarding and other harsh interrogation methods to try to stave off future attacks.

In the hours after the Orlando shooting, Trump issued a statement calling on President Barack Obama to resign for refusing "to even say the words 'radical Islam'" in his response to the attack. He said Clinton should exit the presidential race if she does the same.

In an address from the White House, Obama called the tragedy an act of terror and hate. He did not talk about religious extremists. He said the FBI would investigate the shootings in the gay nightclub as terrorism, but added the gunman's motivations were unclear.

On Monday he said "we're led by a man that either is not tough, not smart, or he's got something else in mind. People cannot believe that President Obama is acting the way he is and cannot even mention radical Islam."

Like Obama, Clinton called the shootings acts of terror and hate, but did not use the words radical Islam in a statement released by her campaign. Instead, she said the country must "redouble our efforts" to defend the country, including "defeating international terror groups, working with allies and partners to go after them wherever they are, countering their attempts to recruit people here and everywhere, and hardening our defenses at home."

Hours after Obama spoke, a law enforcement official confirmed to The Associated Press that Mateen made a 911 call from the club during which he professed allegiance to the leader of the Islamic State group, Abu Bakr al-Baghdadi. The official was familiar with the investigation but not authorized to discuss the matter publicly and spoke on condition of anonymity.

While some Republican leaders have encouraged Trump to abandon his proposed Muslim ban in an effort to broaden his support among voters before November's general election, the Orlando attack appeared Sunday to harden the billionaire businessman's position.

"What has happened in Orlando is just the beginning," Trump tweeted Sunday. "Our leadership is weak and ineffective. I called it and asked for the ban. Must be tough."


Associated Press writer Eric Tucker contributed to this report