Forty nine people were killed and 53 people were injured when a gunman opened fire early Sunday morning at a crowded nightclub in Orlando, Fla.
The casualty figures at the Pulse nightclub make it the worst mass-shooting incident in U.S. history.
The assault was carried out at a gay club during Pride Month, raising immediate questions about whether the gunman was targeting the LGBT community.
Law enforcement sources told CBS News the shooter was Omar Mateen, a U.S. citizen from Port St. Lucie, Fla. He was killed at the scene.
Orlando Police Chief John Mina told reporters early Sunday morning that the suspect, who was killed inside the club during a shootout with police, had an assault-style rifle, a handgun and “some type of device.”
"It appears he was organized and well-prepared," Mina said.
Authorities said the suspect engaged in a shootout with an officer who was working at the club at approximately 2 a.m. before going back into the club and taking hostages. A Special Weapons and Tactics (SWAT) team was sent inside the club to rescue the hostages at approximately 5 a.m.
A trauma surgeon told reporters that many of the injured victims were “critically ill as result of their injuries,” suggesting the death toll could rise even higher.
It was not immediately clear if the incident was inspired by foreign terrorism. An agent from the FBI's Orlando field office said there are “suggestions” that the suspect may have had “leanings” toward radical Islamic ideology.
"We can't say definitively, so we're still running everything to ground," he added.
Sen. Marco RubioMarco Antonio RubioMilley says calls to China were 'perfectly within the duties' of his job Overnight Defense & National Security — Milley becomes lightning rod Joint Chiefs Chairman Milley becomes lightning rod on right MORE (R-Fla.), a member of the Senate Intelligence Committee, said in an interview with CNN that there were “a lot of indications that this has some link to radical Islam.”
Officials did not appear to believe the assault early on Sunday morning was part of a broader attack.
“We feel confident right now there are no other threat to the immediate area or the United States of America,” Ronald Hopper, the FBI’s special agent in charge, said later Sunday morning, adding that the investigation is still ongoing.
President Obama has been briefed on the attack, an aide said in a statement, and ordered federal agencies to provide "any assistance necessary."
Police said a loud noise near the nightclub was a "controlled explosion" used as a distraction when officers breached the building in an armored vehicle and rescued approximately 30 people.
In a Facebook post, the bar and dance club had urged everyone to "get out of pulse and keep running."
"Please keep everyone in your prayers as we work through this tragic event," another Pulse Facebook post said. "Thank you for your thoughts and love."
"Shooter opened fire @ around 2:00am," one commenter on the club's Facebook page said. "People on the dance floor and bar got down on the floor and some of us who were near the bar and back exit managed to go out through the outdoor area and just ran. I am safely home and hoping everyone gets home safely as well."
The Orlando Regional Medical Center, located a few miles away, was placed on lockdown and only essential workers were being allowed inside, CNN reported. Arnold Palmer Hospital and Winnie Palmer Hospital were also placed on lockdown out of caution.
— Julian Hattem and Rebecca Savransky contributed to this report, which was updated on June 13 at 7:58 a.m.