ORLANDO: Fifty people died when a heavily-armed gunman opened fire and seized hostages at a popular gay nightclub in Orlando, Florida in the worst mass shooting in US history.

Who was the gunman?

US media quoting law enforcement sources widely identified the gunman, who died in a shootout with police, as Omar Mateen, 29.

A US citizen, Mateen was born to Afghan parents living in New York and resided in Port St Lucie, Florida, about two hours drive from Orlando.

CBS News reported that Mateen has no apparent criminal history but authorities were looking into whether the suspect had “leanings” toward Islamic extremism.

Police said they had identified the shooter but were yet to officially name him, pending notification of his next of kin.

They recovered from the suspect a handgun, an AR-15 type assault rifle and an unknown number of additional rounds.

What was his target?

The Pulse nightclub targeted in the assault is a popular dance club and bar known on the Orlando LGBT party scene for its drag shows. It is not known exactly how many people were inside at the time of the attack, but it was packed with Saturday night revelers come to watch a drag queen contest.

The nightclub was co-founded by a woman whose brother died in 1991 after battling HIV, and called itself “more than 'just another gay club.'”

In 2004, Barbara Poma and a friend opened the club Pulse to keep her brother's spirit alive, and named it in honor of his heartbeat.

The Pulse club has become a flagship establishment for the LGBT community in Florida and further afield and forms part of a vibrant activist scene that aims to promote awareness of gay rights.

What was his motive?

Police are still working to determine what led the gunman to unleash carnage in Orlando.

The FBI is investigating the attack as an act of terrorism and says it is investigating the possible radical Islamic “leanings” of the suspect.

CNN and NBC cited law enforcement sources as saying that Mateen made a 911 phone call shortly before the shooting pledging allegiance to the militant Islamic State (IS) group.

But the suspect's father told NBC news his son may have been motivated by homophobia and not by his Muslim faith.

Mir Seddique recalled a recent incident in downtown Miami when his son became angry after witnessing a gay couple embracing.

The Pulse attack coincided with gay pride month in the United States, with festive marches and events being held all over the country including in Orlando last week.