A 29-year-old man killed at least 50 people and wounded 53 others at an Orlando nightclub early Sunday morning in the worst mass shooting in US history.
President Barack Obama said from the White House that the shooting was an act of "terror and hate," and represented an attack on all Americans.
"We are still learning all the facts, this is an open investigation" the president said. "I've reached no definitive judgment on the motivations of the killer. The FBI is investigating this as an act of terror."
The gunman, identified by law enforcement officials as Omar Siddiqui Mateen, entered Pulse, a popular gay dance club in downtown Orlando, just after 2:00am and began shooting patrons using an AR-15-style assault rifle and a handgun. He then took hostages and remained in a standoff with police unit they stormed the club at around 5:00am and killed him in a gun battle. Pulse was crowded with some 350 revelers on a Latin music night when the attack happened.
A lengthy exchange of gunfire could be heard from outside the club in one video posted on Twitter at 5:40am.
"At first we thought it was the DJ adding shots into the set-list… and when we realized it was something different people were just ducking and screaming," said Luis Burbano, a witness who told ABC News he was bartending at the club last night when the shooting started. "The gunshots got closer… there was a break after 10 to 20 gunshots… and we started jetting up an exit towards the stage, and then the shooting started again, and they were getting louder."
"I heard at least 20 to 30 rounds by the time I got out," Burbano said.
Joshua McGill, who was at the club when the shooting started, described how he fled the attack in a post on Facebook.
"I hid under a car and found one of the victims that was shot," McGill said. He described using his shirt to bandage one victim, and recalled dragging the person to a nearby police officer. "Words cannot and will not describe the feeling of that," he said. "Being covered in blood. Trying to save a guy's life."
Other images on social media showed chaotic scenes as first responders treated wounded victims.
No police officers were seriously injured in the raid on the club, but one was hit in the helmet with a bullet.
NBC News reported that several unnamed law enforcement officials said Mateen pledged allegiance the Islamic State group in a call to 911 before the attack.
Ron Hopper, the FBI agent overseeing the agency's Tampa field division, told reporters that Mateen made a call to 911 early Sunday morning that was "general to the islamic State," but said he hadn't heard the tape and couldn't confirm that Mateen had pledged allegiance to the militant group in the call.
Hopper also said the FBI had interviewed Mateen at least three times in the past five years about suspected terrorist connections, but he was not under surveillance or investigation at the time of Sunday's attack. He also said Mateen legally purchased at least two of the weapons used in the attack in the past week.
After reports of Mateen's pledge of allegiance emerged, the Islamic State's official Amaq news agency said that Mateen was an "Islamic State fighter" and that the group was responsible for the attack.
"The armed attack that targeted a gay night club in the city of Orlando in the American state of Florida which left over 100 people dead or injured was carried out by an Islamic State fighter," the message said.
It's still unclear whether the Islamic State supported Mateen, encouraged him to mount the attack, or if the group was even in direct contact with him.
Mateen was born in the US in 1986 to parents from Afghanistan, and lived in Port St. Lucie, Florida, about a two-hour drive southeast of Orlando. The Orlando police department released a photo of him on Sunday afternoon.
Hopper, the FBI agent, said Mateen was questioned twice in 2013 for comments he made at work about possible contacts with terrorists, and in 2014 for possible ties to an American suicide bomber.
"The FBI investigated and interviewed Mateen, and found contact was minimal and that he was not a threat at that time," Hopper said.
According to Reuters, three US officials familiar with the massacre investigation said that no evidence had yet been found of a direct link with the Islamic State or any other militant group.
There is "no evidence yet that this was directed or connected to ISIS. So far as we know at this time, his first direct contact was a pledge of bayat (loyalty) he made during the massacre," a US counterterrorism official said, referring to the 911 call the suspect made on Sunday.
Another US intelligence official told Reuters that the Islamic State may have quickly claimed responsibility partly because the group has been losing fighters and territory in Iraq and Syria.
"The fact that a website connected to Daesh applauded it doesn't mean anything," the official said, using an Arabic acronym for the militant group. "They are losing on their home turf, and it's not surprising if they're looking for some kind of twisted victory."
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Reuters contributed to this report.