Militant group Islamic State (IS) has claimed responsibility for the massacre in Orlando, Florida, but US officials say there's no immediate evidence to prove the connection.
A gunman killed at least 50 people and injured 53 more in an attack on the Pulse nightclub on Sunday (local time), before being shot dead by police.
The media arm of IS, Amaq, ran a statement claiming responsibility for the attack.
"The armed attack that targeted a gay nightclub 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," said the statement.
The shooter has been identified as 29-year-old Omar Mir Sadiq Mateen. He was an American citizen born in New York, whose parents were from Afghanistan.
An FBI official said they first became aware of him in 2013 after he made inflammatory comments to co-workers. He was investigated, but cleared.
He was investigated again in 2014, because of his connection with American suicide bomber Moner Mohammad Abusalha. Again, he was deemed not to be a threat.
Law enforcement officials say he claimed allegiance to IS (also known as ISIS) in 911 calls he made before Sunday's attack. But officials say proving the suspected link to radical Islamism will require further investigation.
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," said a US counter-terrorism official, speaking on condition of anonymity.
A US intelligence official said it was not unexpected that IS, also referred to as Daesh, would claim responsibility, given the group has been suffering serious losses in Iraq and Syria.
"The fact that a website connected to Daesh applauded it doesn't mean anything," said the US intelligence official, using an Arabic language acronym for Islamic State.
"They are losing on their home turf, and it's not surprising if they're looking for some kind of twisted victory."
America's Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) says Mateen legally bought both weapons used in the attack -- a handgun and a long gun. He purchased them in Florida within the past week.