Fifty people are dead and 53 more are in critical condition after a mass shooting at an Orlando gay nightclub early this morning. A surgeon at Orlando Regional Medical Center said the death toll was likely to rise.

This is the deadliest mass shooting in U.S. history.

Police have identified the shooter as Omar Mateen, a 29-year-old Florida man who works as a security guard. He was armed with an assault-type rifle and handgun, which ATF officials said he purchased legally within the last few weeks.

The shooting began at 2 a.m. Eastern time at Pulse, a popular gay nightclub in Orlando. More than 300 people were inside at the time of the shooting. Witnesses said at first they thought the gunfire was part of the music.

The incident became a hostage standoff. A SWAT unit entered the club around 5 a.m., and Mateen was killed in the ensuing gunfight.

"There's blood everywhere," Orlando Mayor Buddy Dyer said in a press conference Sunday morning.

Authorities have not yet confirmed a motive for the shooting, but President Obama said it is being investigated as an act of terrorism.

"We know enough to say that this was an act of terror and an act of hate," he said.

NBC News reported that at some point before or during the shooting, Mateen made a 911 call in which he pledged allegiance to ISIS. At around 11:30 Sunday morning, an ISIS-affiliated news agency claimed the attack was carried out by an Islamic State fighter.

However, Rep. Adam Schiff, Chairman of the House Intelligence Committee, said that a direct connection between Mateen and the Islamic State has not been identified by federal law enforcement.

FBI Assistant Special Agent in Charge Ron Hopper said the FBI investigated Mateen in 2013 for statements he made to coworkers. The investigation found "nothing of substance."

Hopper said Mateen was again on the FBI's radar in 2014, for connections to an American suicide bomber.

Mateen's father told NBC News he didn't know of anything his son was planning, but he didn't think it had to do with religion. "We are apologizing for the whole incident … We are in shock like the whole country."

He added that during a recent trip to Miami, Mateen was angered by gay couple kissing.

The shooting occurs during Pride Month, with many Pride festivals and parades happening across the nation.

In a statement, Vice President Joe Biden called the apparent targeting of the LGBT community "evil and abhorrent."

Seattle Mayor Ed Murray, who is openly gay, called the incident "absolutely an attempt at intimidation and fear." He said he believes the LGBT community would not be intimidated.

Reach Executive Producer Sam Bergum here, or follow her on Twitter.

Updated June 12, 5:10 p.m. Pacific time: This post was updated to include statements by Vice President Joe Biden and Seattle Mayor Ed Murray as well as new information from the Associated Press.