Evangelist Franklin Graham has criticized U.S. President Obama for failing to refer to the Orlando massacre as "Islamic terrorism" even though the shooter, Omar Mateen, pledged allegiance to ISIS moments before the attack and claimed to have been inspired by radical Islamic ideology. The attack on an openly gay nightclub left least 49 people dead and and 53 more wounded, making it the deadliest mass shooting in American history. 

"I appreciate President Barack Obama speaking to the nation yesterday in the wake of the Orlando massacre," Graham, the leader of the Billy Graham Evangelistic Association and Samaritan's Purse, said in a Facebook message on Monday. "He was right in saying that this tragic shooting was an act of terror and hate. But why didn't he say it was an act of radical Islamic terrorism? Mr. President, with all due respect, don't forget Fort Hood; don't forget the Boston Marathon; don't forget San Bernardino; and don't forget 9/11. What do they all have in common? They were all Islamic terrorists carrying out their treachery in the name of their religion. "

Speaking on Sunday, Obama called the shooting an "act of terror" that served as a "sobering reminder that attacks on any American, regardless of race, ethnicity, religion or sexual orientation is an attack on all of us."

In remarks from the White House briefing room, the President said, "No act of hate or terror will ever change who we are or the values that make us Americans."

Obama also called for tighter gun control laws, saying the Orlando shooting rampage is a reminder of how easy it is for someone to get a hold of a weapon that could kill people in a "school, or a house of worship, or a movie theater, or in a nightclub."

"And we have to decide if that's the kind of country we want to be," he added. "And to actively do nothing is a decision as well."

Mateen, 29, an American citizen who was shot dead by a SWAT team after the shooting spree, was investigated by the FBI back in 2013 after making "inflammatory comments" to co-workers alleging possible "terrorist ties."

The following year, the FBI investigated him once again because of ties with an American who traveled to the Middle East to become a suicide bomber.

Law enforcement sources told NBC News he swore allegiance to the leader of the Islamic State group, Abu Bakr al-Baghdadi, in a 911 call made 20 minutes before the rampage at Pulse. He also reportedly mentioned the Boston Marathon bombers, a U.S. official said. Matten's family maintains the attack had "nothing to do with religion" and said the shooter wasn't particularly religious, according to CNN.

However, the Islamic State's Amaq news agency claimed responsibility for the attack Sunday, and a message posted on a site associated with the ISIS news agency Amaq described the attacker as "an Islamic State fighter."

Continuing his Facebook post, Graham, whose father is the world-renowned preacher Billy Graham, wrote, "Mr. President, your administration has cut our military to the bone and some experts say we're the weakest we have been in modern times. It's time we were rebuilding our military force worldwide to take on this enemy. It's time we tighten America's borders until we can clearly vet those we're letting into the country. Mr. President, how many more American lives have to be lost before you understand the threat radical Islam is to every freedom-loving person?"

This is not the first time Graham has criticized Obama for refusing to acknowledge the threat of radical Islam. Last year, he told Fox News host Greta Van Susteren that Obama, whose "entire life" was shaped by Islam, cannot accept the evil in front of him and apparently is trying to "protect Islam,"

"We have to understand who we're dealing with and you have to deal with it accordingly," he said during an On the Record interview. "The president is ignoring the fact that these are Islamic extremists, these are terrorists, and ISIS is a terrorist organization and they are bent on destroying the West. They've already said they want the flag of Islam to fly over the White House."

"The president, his entire life, his whole influence has been Islam," said Graham. "His mother was married to a Muslim. His father's a Muslim. Then she married a [Muslim] man from Indonesia."

"He [Obama] was raised in Indonesia," said the evangelist. "He went to Islamic schools. I assume she was a Muslim. So, his whole life experiences have been surrounded by Islam. He only knows Islam. And he's given a pass to Islam. He's refusing to accept and understand the evil that is in front of him."

Republican nominee Donald Trump also criticized Obama following Sunday's attacks and called for the President to step down from his position after failing to address radical Islam.

"President Obama disgracefully refused to even say the words 'Radical Islam,'" Trump said in the statement. "For that reason alone, he should step down. If Hillary Clinton, after this attack, still cannot say the two words 'Radical Islam' she should get out of this race for the Presidency."