Washington-In the biggest massacre in the history of the United States, the Federal Bureau of Investigation announced on Sunday that Omar Mateen, a U.S. citizen of Afghan descent, had carried out the attack at the gay nightclub in Florida, leaving at least 50 people dead.

The FBI said Mateen apparently made a 911 call just before the massacre to claim allegiance to ISIS. Minutes after the police announcement that the 29-year-old made the call, U.S. President Barack Obama condemned the “horrific massacre.”

“This was an act of terror and an act of hate,” Obama said in a somber White House address to mark the worst mass shooting in U.S. history.

“As Americans, we are united in grief, in outrage, and in resolve to defend our people,” he said.
The president also praised Orlando’s police.

He said the U.S. will continue its operations against ISIS and the rest of the terrorist organizations.

“This is a sobering reminder that attacks on any American … is an attack on all of us,” he said.

Orlando Mayor Buddy Dyer announced during a news briefing the number of casualties.

Fifty people died and another 53 were injured, he said.

He urged U.S. citizens to donate blood at Orlando hospitals.

Then spoke Orlando Police Chief John Mina, who said that security forces received appeals from inside the nightclub where they were involved in a shooting outside the facility after rushing to the area. He did not provide any explanation as to why the police entered the nightclub after three long hours.

Republican Sen. Marco Rubio told CNN that the investigations will reveal If the suspect acted alone or had ties to terrorists.

Extremists do not sympathize with gays, he said, adding all religions have violent extremists.

New York GOP Rep. Peter King told CNN that Mateen did not act alone. But he refused to give further details.

Presidential candidates Hillary Trump and Donald Trump also tweeted, condemning the attack.