The self-proclaimed Islamic State has claimed responsibility for the deadliest mass shooting in United States history, which has left at least 50 dead and dozens more injured.
Authorities say 29-year-old Omar Mateen opened fire in a gay nightclub in the Florida city of Orlando before being shot dead by police.
US president Barack Obama has described the killings as an act of terror and hate.
Once again, the United States has been left reeling by a shooting that has claimed dozens of lives in just minutes.
Police believe a man named Omar Mateen travelled several hours from his Florida home to the Pulse nightclub in Orlando and opened fire.
After a three-hour hostage stand-off, police stormed the venue, shooting dead the 29-year-old, but not before he had killed at least 50 people.
A bartender inside the packed club, Tiffany Johnson, has described the scene.
"All of a sudden, we heard like a 'boom, boom,' and we thought it was, like, the stereo. We were, like, 'Oh, my goodness, is something going on with the ...' -- you know, just the music. And, after the second shot, there was a pause, and then it just started -- shooting, shooting, shooting, shooting. I just ... I ran. I just was trying to get out of there as soon as possible. I just didn't look back, just ran."
Patron Christopher Hansen says he heard the gunshots and turned to see the patron next to him screaming.
"The person I was with was shot in the back. I had to take my bandana off and tie it up and put it in the bullet hole that was in his back so that he wasn't bleeding, because he was bleeding so bad his whole pant leg was red. It was so soaked. My hands were just covered."
Not for the first time in his presidency, Barack Obama came forward to lead the nation in mourning the victims of yet another gun massacre.
"We pray for the families who are grasping for answers with broken hearts. We stand with the people of Orlando, who have endured a terrible attack on their city. Although it's still early in the investigation, we know enough to say that this was an act of terror and an act of hate, and, as Americans, we are united in grief and outrage and in resolve to defend our people."
In the aftermath of the shooting, the self-proclaimed Islamic State has claimed responsibility, saying Omar Mateen was one of its fighters.
But US officials are warning that clearly defining his ties requires further investigation.
Mateen's Afghanistan-born father has revealed his son's homophobia and how he only recently expressed disgust at witnessing two men kissing on the streets of Miami.
In the Fort Pierce neighbourhood where Mateen grew up, those who knew him say there was no hint of militancy or homophobia.
One, Ryan Jones, described him as cool, calm and collected.
"He was never an angry monster. I never saw him in a fistfight my entire life. (He) loved to hang out with women and party with young kids our age. He was a normal teenage person and lived a normal life. Never crazy. Never."
But FBI investigator Ronald Hopper has revealed Mateen had first come to the attention of the FBI three years ago after making threatening comments to co-workers.
"The FBI thoroughly investigated the matter, including interviews of witnesses, physical surveillance and records checks. In the course of the investigation, Mateen was interviewed twice. Ultimately, we were unable to verify the substance of his comments, and the investigation was closed."
Despite that, and further FBI scrutiny a year later, he was still able to get a gun permit.
And in the middle of a US presidential campaign, there is already renewed debate about the US firearm culture.
Barack Obama is not shying away from the controversy.
"Today marks the most deadly shooting in American history. The shooter was apparently armed with a handgun and a powerful assault rifle. This massacre is, therefore, a further reminder of how easy it is for someone to get their hands on a weapon that lets them shoot people in a school, or in a house of worship, in a movie theatre or in a nightclub. And we have to decide if that's the kind of country we want to be."
Democratic presidential hopeful Bernie Sanders has sent his sympathies and says the country must get tougher on gun control.
"It's horrific, it's unthinkable. And our ... Oh, it's horrific, it's unthinkable, and our just hopes go out to all those who were shot that they can recover. And I got to tell you, you know, over 25 years ago, I believed that, in this country, we should not be selling automatic weapons which are designed to kill people."
Republican Donald Trump was typically combative, thanking supporters for congratulating him about his terrorism warnings regarding Muslims and urging toughness and vigilance.
He tweeted again later, calling for a ban on Muslims entering the United States.
Rival Hillary Clinton pledged her support for the LGBTI community in the wake of the massacre.
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