As hundreds gathered early Sunday at a popular gay nightclub in Orlando, Florida, police say a gunman opened fire – leaving at least 49 people dead and injuring at least 53 more, in what is now the deadliest mass shooting in American history.
“I can’t stop crying,” witness Carmen Pena, 30, tells PEOPLE. “I feel so sick. It was awful.”
Hours later, the Islamic State claimed responsibility for the attack, Reuters reports. In a Sunday afternoon news conference, U.S. Sens. Bill Nelson and Marco Rubio also laid blame with ISIS.
Police say the shooter, reportedly identified as Omar Saddiqui Mateen, opened fire just after 2 a.m. local time at Pulse, a popular gay club south of downtown Orlando founded as a place of “acceptance and love.”
An officer working security off-duty at the club initially responded to the shooting, “engaging in the gun battle with the suspect” before the suspect went back into the club, creating a hostage situation, Orlando Police Chief John Mina said at a Sunday morning news conference.
The gunman was killed after 5 a.m. local time, in an exchange of gunfire with authorities, as they moved in to rescue the hostages.
Mina said he “appeared to be carrying an assault-type rifle and a handgun. He had some type of device on him.” Mina said he was not from the area.
Authorities confirmed the increased casualties Sunday morning, having initially said at least 20 people were killed. One officer was injured, but “his Kevlar helmet saved his life,” Mina said.
Authorities investigative possible ISIS connection
“Agents in Afghanistan are looking at the shooter’s connections there,” an FBI source with knowledge of the investigation tells PEOPLE. “He has family there. This is not to say his family is connected to the shootings. This is the normal course of an investigation like this. We ask questions.”
“We are examining the shooter’s social media activity,” the source says. “Did he have connections to radical factions? Was he inspired?”
“This is a priority.”
“The Islamic State called for attacks on the West during Ramadan,” the source continues. “In the wake of San Bernardino, we are very much mindful of how this can play out.”
“The big concern now is to find out if the shooter had accomplices,” the source says.
A federal law enforcement official also tells PEOPLE that the shooter spoke with a 911 operator during his attack, pledging allegiance to ISIS and mentioning the Tsarnaev brothers, who orchestrated the Boston Marathon bombings in 2013.
“The FBI is appropriately investigating this as an act of terror,” he said. “We will go wherever the facts lead us … What is clear is he was a person filled with hatred.”
Anyone with information about the shooting is urged to call 1-800-CALL-FBI
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Witnesses describe the scene: Bullets, blood and bravery
According to the club’s Facebook page, Saturday was the club’s “Latino night.”
“Calling all our Latinos, Latinas & everyone that loves a little Latin flavor,” the club wrote on their page ahead of the shooting. “It’s time to party tonight!
During the shooting, someone posted from the club’s page telling everyone to flee:
“Everyone get out of pulse and keep running.”
Witnesses have described the shooting scene to PEOPLE, from both inside and around the nightclub, as “crazy” and “awful.”
Pena tells PEOPLE she was inside the club, in the same room with the shooter, when he started firing.
“I know that he was in the room, because I heard a woman screaming, and then all of a sudden she stopped,” Pena says. “The gunshots were so loud. It felt like they were right next to my ears. Every second, I thought I was going to be hit.”
Twenty-nine-year-old Antonio (no last name given) tells PEOPLE he was outside Pulse with his friends when the shooting started.
“We were just leaving,” he says. “We heard this sound that I thought was the beat of the music inside. And then people started running out the front door. All hell broke loose.”
Along with the screams outside the club, Antonio says he saw one woman “who was holding her shoulder and her hand was covered in blood, she wasn’t screaming or making any noise, just walking out staring straight ahead,” as another woman helped her walk.
He says he saw a man stepped on as he tripped and fell. And he says he saw a man who had run outside stop and scream back at the door, “Michael! Michael!”
Outside the club, the victims tended to one another, Antonio says.
“A guy walked past and said, ‘Help, help. I think I’ve been shot,’ ” he says. “These other guys ran up and tried to stop the bleeding. They were putting their clothes on it to stop the bleeding.
“People were taking off their shirts and using them to stop the bleeding.”
And in the parking lot, Antonio says, “a guy was lying on the ground and two women were kneeling over him, telling him, ‘Stay with me. Stay with me.’ ”
Family members of the victims can call 407-246-4357 to find out more information.
So far only six victims have officially been identified: Edward Sotomayor Jr., 34; Stanley Almodovar III, 23; Luis Omar Ocasio-Capo, 20; Juan Ramon Guerrero, 22; Eric Ivan Ortiz-Rivera, 36, and Peter O. Gonzalez-Cruz, 22. Authorities confirmed that all six men had died.
An online list is being updated as authorities make contact with the victims’ families.
• With reporting by SUSAN KEATING