Presumptive Republican presidential nominee Donald Trump attacked President Obama for refusing to attribute the Orlando, Fla., nightclub shooting to radical Islam.

"In his remarks today, President Obama disgracefully refused to even say the words 'Radical Islam'. For that reason alone, he should step down," Trump said in a statement Sunday afternoon.

"If Hillary Clinton, after this attack, still cannot say the two words 'Radical Islam' she should get out of this race for the Presidency," he added.

In the statement and on Twitter, Trump again focused on immigration.

"We admit more than 100,000 lifetime migrants from the Middle East each year. Since 9/11, hundreds of migrants and their children have been implicated in terrorism in the United States," Trump said in the statement.

The shooting at Pulse, a gay nightclub, is currently being characterized as an act of domestic terrorism. The suspect, who was killed inside the club after killing 49 and injuring 53 others, had an assault-style rifle, a handgun and "some type of device."

The FBI confirmed on Sunday it had interviewed the suspect, Omar Mateen, in 2013 when he made "inflammatory comments to coworkers alleging possible terrorist ties."

In the course of that investigation, the New York-born 29-year-old was interviewed twice, but the FBI was unable to verify the substance of his comments.

Again in 2014, the FBI conducted an investigation into possible ties between Mateen and an American suicide bomber, it said Sunday, but determined that the contact did not constitute a threat at that time.

In a statement earlier Sunday, President Obama called the mass shooting "an act of terror and hate," vowing a swift federal investigation into the attack.

"This was an act of terror and an act of hate," Obama told reporters in the press briefing room. "And as Americans, we are unified in grief and outrage and in resolve to defend our people."

But that did not go far enough for Trump.

"If we do not get tough and smart real fast, we are not going to have a country anymore. Because our leaders are weak," he said in his statement. "I said this was going to happen - and it is only going to get worse. I am trying to save lives and prevent the next terrorist attack. We can't afford to be politically correct anymore."

Trump promised to discuss the shooting, immigration and national security on Monday, his campaign announced Sunday evening.

A speech was already planned at St. Anselm College's New Hampshire Institute of Politics and was meant to focus solely on Clinton, but it will now take place in the shadow of the worst mass shooting in American history.