A gunman has killed 50 people overnight at a gay nightclub in Florida before being killed by police in the deadliest mass shooting in U.S. history.
Orlando’s Mayor Buddy Dyer said on June 12 that another 53 people were hospitalized after the attack at the Pulse club.
President Barack Obama said Americans would stand united and "not give in to fear or turn against each other" in the wake of "an act of terror and an act of hate."
A "definitive judgement" about the motives of the gunman were not yet available, said Obama, who ordered flags to be flown at half-staff at the White House and federal buildings.
Police entered the Pulse club some three hours after the shooting unfolded, and killed the gunman who had taken hostages.
Police described the attack as an act of terrorism.
The attacker was carrying an assault rifle and a handgun, Orange County Sheriff Jerry Demings said.
The shooter was identified by authorities as Omar Mateen, whom media reports described as a U.S. citizen of Afghan descent.
The Amaq news agency, which supports the extremist Islamic State (IS) group, said an IS "fighter" had carried out the shooting.
It did not specify whether IS was directly involved or simply taking credit for inspiring the attack.
U.S. media reported that Mateen, described as a 29-year-old Florida resident born in New York, had been licensed as a private security officer.
WATCH: Police Search Home Of Orlando Shooter (Reuters video)
Reports also said Mateen called 911 before the shooting and swore allegiance to the IS group.
However, U.S. officials said they had seen no immediate evidence of any direct connection between the shooting and the IS group or any other foreign extremist group.
The father of the suspect said he believed his son was motivated by hatred of gays -- not by his Muslim religion.
"This had nothing to do with religion," Mir Seddique told NBC News.
Seddique said his son got angry when he saw two men kissing in Miami a couple of months ago and thinks that may be related to the shooting.
"We are saying we are apologizing for the whole incident,” he also said. “We are in shock like the whole country."
In a statement, Afghan President Ashraf Ghani condemned the attack, saying that "targeting civilians is not justifiable under any circumstances whatsoever."
"We are sickened and heartbroken by this appalling attack," said Nihad Awad, executive director of a leading Muslim organization in the United States -- the Council on American-Islamic Relations.
Law enforcement officer Danny Banks said authorities are looking into whether the shooting was an act of domestic or international terror, and if the shooter was acting alone.
FBI agent Ronald Hopper said the attacker appeared to "have leanings toward" radical Islamist ideology.
The FBI said agents investigated the man twice but closed those cases after interviewing him. Agent Hopper said Mateen was first investigated in 2013 after he made inflammatory comments to coworkers alleging possible ties to terrorists. In 2014, the agency looked into potential ties connecting Mateen to Moner Mohammad Abusalha, the first American to carry out a suicide attack in Syria.
The death toll given by Mayor Dyer means that the Orlando attack surpasses the 2007 massacre at the Virginia Tech college that left 32 people dead.
"Today we're dealing with something that we never imagined and is unimaginable," Dyer said, adding that there was "an enormous amount of havoc" and "blood everywhere."
Also on June 12, police in Santa Monica, California, arrested a "heavily armed" man overnight who said he had planned to go to a gay pride rally in Los Angeles.
The incident appeared to be "completely unrelated" to the Orlando shooting, Los Angeles Mayor Eric Garcetti said.
With reporting by Reuters, AP, and AFP