ORLANDO, Fla. — The massacre at an Orlando nightclub that claimed 50 lives is the worst mass shooting in U.S. history.

Police Chief John Mina confirmed at a news conference Sunday morning that 50 people were killed, up from 20 as earlier reported.

Mayor Buddy Dyer says 53 more are hospitalized after the early Sunday incident. He says the shooter is among the dead. He also says the shooter used an assault rifle on all those dead. Officials say one officer was shot, and has injuries to his face.

Earlier, U.S. Rep. Alan Grayson identified the shooter as Omar Mateen of Port St. Lucie, Florida. He cited law enforcement officials in speaking to reporters.

Officials also have said they’re investigating whether the incident was an act of terrorism.

Police Chief John Mina said the shooter also had some sort of “suspicious device.” He said the suspect exchanged gunfire with an officer working at the gay club around 2 a.m., then went back inside and took hostages among more than 300 people inside.

Around 5 a.m., authorities sent in a SWAT team to rescue the hostages. Police have not determined an exact number of casualties, but Mina said “approximately 20” bodies were inside the club.

Authorities were looking into whether the attack was an act of domestic or international terror, and if the shooter was a lone wolf, according to Danny Banks, an agent with the Florida Department of Law Enforcement.

“This is an incident, as I see it, that we certainly classify as domestic terror incident,” Orange County Sheriff Jerry Demings said.

FBI agent Ron Hopper said there was no further threat to Orlando or the surrounding area.

When asked if the gunman had a connection to radical Islamic terrorism, Hopper said authorities “have suggestions that individual has leanings towards that.”

Police had said previously on Twitter that there was a “controlled explosion” at the scene of the shooting at Pulse Orlando, a dance club. Mina said that noise was caused by a device intended to distract the shooter.

Mina Justice was outside the club early Sunday trying to contact her 30-year-old son, Eddie, who texted her when the shooting happened and asked her to call police. He told her he ran into a bathroom with other club patrons to hide. He then texted her: “He’s coming.”

“The next text said: ‘He has us, and he’s in here with us,’” she said. “That was the last conversation.”

Dozens of police vehicles swarmed the area around the club. At least two police pickup trucks were seen taking what appeared to be shooting victims to the Orlando Regional Medical Center.

Pulse Orlando posted on its own Facebook page around 2 a.m.: “Everyone get out of pulse and keep running.” Just before 6 a.m., the club posted an update: “As soon as we have any information, we will update everyone. Please keep everyone in your prayers as we work through this tragic event. Thank you for your thoughts and love.”

Police said local, state and federal agencies were investigating.

The attack follows the fatal shooting late Friday of 22-year-old singer Christina Grimmie, who was killed after her concert in Orlando by a 27-year-old Florida man who later killed himself. Grimmie was a YouTube sensation and former contestant on “The Voice.”

Jon Alamo said he was at the back of one of the club’s rooms when a man holding a weapon came into the front of the room.

“I heard 20, 40, 50 shots,” Alamo said. “The music stopped.”

Club-goer Rob Rick said the shooting started just before closing time.

“Everybody was drinking their last sip,” he said.

He estimated more than 100 people were still inside when he heard shots, got on the ground and crawled toward a DJ booth. A bouncer knocked down a partition between the club area and an area in the back where only workers are allowed. People inside were able to then escape through the back of the club.

By MIKE SCHNEIDER, Associated Press

Eds: Raises mention of authorities investigating attack as terrorism. Adds more video.

ORLANDO, Fla. (AP) — A gunman wielding an assault-type rifle and a handgun opened fire inside a crowded Florida nightclub before dying in a gunfight with SWAT officers, police said Sunday. The attack left about 20 people dead and 42 others wounded.

Authorities were investigating it as an act of terrorism.

Police Chief John Mina said the shooter also had some sort of “suspicious device.” He said the suspect exchanged gunfire with an officer working at the gay club around 2 a.m., then went back inside and took hostages among more than 300 people inside.

Around 5 a.m., authorities sent in a SWAT team to rescue the hostages. Police have not determined an exact number of casualties, but Mina said “approximately 20” bodies were inside the club.

Authorities were looking into whether the attack was an act of domestic or international terror, and if the shooter was a lone wolf, according to Danny Banks, an agent with the Florida Department of Law Enforcement.

“This is an incident, as I see it, that we certainly classify as domestic terror incident,” Orange County Sheriff Jerry Demings said.

FBI agent Ron Hopper said there was no further threat to Orlando or the surrounding area.

When asked if the gunman had a connection to radical Islamic terrorism, Hopper said authorities “have suggestions that individual has leanings towards that.”

Police had said previously on Twitter that there was a “controlled explosion” at the scene of the shooting at Pulse Orlando, a dance club. Mina said that noise was caused by a device intended to distract the shooter.

Mina Justice was outside the club early Sunday trying to contact her 30-year-old son, Eddie, who texted her when the shooting happened and asked her to call police. He told her he ran into a bathroom with other club patrons to hide. He then texted her: “He’s coming.”

“The next text said: ‘He has us, and he’s in here with us,’” she said. “That was the last conversation.”

Dozens of police vehicles swarmed the area around the club. At least two police pickup trucks were seen taking what appeared to be shooting victims to the Orlando Regional Medical Center.

Pulse Orlando posted on its own Facebook page around 2 a.m.: “Everyone get out of pulse and keep running.” Just before 6 a.m., the club posted an update: “As soon as we have any information, we will update everyone. Please keep everyone in your prayers as we work through this tragic event. Thank you for your thoughts and love.”

Police said local, state and federal agencies were investigating.

The attack follows the fatal shooting late Friday of 22-year-old singer Christina Grimmie, who was killed after her concert in Orlando by a 27-year-old Florida man who later killed himself. Grimmie was a YouTube sensation and former contestant on “The Voice.”

Jon Alamo said he was at the back of one of the club’s rooms when a man holding a weapon came into the front of the room.

“I heard 20, 40, 50 shots,” Alamo said. “The music stopped.”

Club-goer Rob Rick said the shooting started just before closing time.

“Everybody was drinking their last sip,” he said.

He estimated more than 100 people were still inside when he heard shots, got on the ground and crawled toward a DJ booth. A bouncer knocked down a partition between the club area and an area in the back where only workers are allowed. People inside were able to then escape through the back of the club.

Christopher Hansen said he was in the VIP lounge when he heard gunshots. He continued to hear shooting even after he emerged, where police were telling people to back away from the club. He saw injured people being tended to across the street.

“I was thinking, ‘Are you kidding me?’ So I just dropped down. I just said, ‘Please, please, please, I want to make it out,’” he said. “And when I did, I saw people shot. I saw blood. You hope and pray you don’t get shot.”