"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.
"So this is a sobering reminder that attacks on any American - regardless of race, ethnicity, religion or sexual orientation - is an attack on all of us and on the fundamental values of equality and dignity that define us as a country."
Chilling details have emerged of the murderous rampage that targeted a gay nightclub frequented by Orlando's LGBT community.
DJ Ray Rivera, 42, was playing reggae music on the patio when the shooting started. "I thought it was firecrackers," Rivera said. "I saw bodies on the floor, people on the floor everywhere."
Pulse posted a message on Facebook urging party-goers to "get out and keep running" as bullets started to fly. About 320 people were inside the club at the time and about 100 people were taken hostage.
Around 5am authorities sent in a Swat team to rescue the hostages, using a "controlled explosion" to distract Mateen before fatally shooting him and rescuing about 30 hostages who were hiding in a bathroom.
The federal agency confirmed yesterday that Mateen was first interviewed in 2013 after he made "inflammatory remarks" to a colleague, alleging "possible terrorist ties".
"The FBI thoroughly investigated the matter including interviews of witnesses, physical surveillance and records checks," said Ron Hopper, an FBI special agent. "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."
In 2014, Mateen came to the FBI's attention again and agents interviewed him about a potential connection with American suicide bomber Moner Abu Salha.
"We determined that contact was minimal and didn't constitute a substantive relationship or threat at that that time," Mr Hopper said.
The Bureau of Alcohol, Tobacco and Firearms confirmed Mateen had bought two weapons legally within the last week.
"He is not a prohibitive person so he can legally walk into a gun dealership," a spokesman said.
Mateen's former employer, global security firm G4S, confirmed he had worked for the company since 2007.
In New Zealand flags flew at half mast yesterday at the American Embassy in Wellington.
Prime Minister John Key said he had written to President Obama to express New Zealand's condolences over the "shocking" mass shooting.
The Ministry of Foreign Affairs and Trade confirmed that no New Zealanders were believed to have been caught up in the shootings.
A New Zealander living in Orlando said she had considered moving home after two Florida shootings in as many days. Claire North, who has lived in the US state for 21 years, spent much of yesterday following the mass shooting on the news.
"It's on all the channels; you can't get away from it. It's pretty shocking."
The FBI believed Mateen - a Muslim and father to a 3-year-old son - may have "leanings to radical Islamic terrorism".
But his father, Mir Seddique, told NBC News his son became angry when he saw two men kissing in Miami several months ago.
"This has nothing to do with religion," he said.
"We were in Downtown Miami, Bayside, people were playing music. And he saw two men kissing each other in front of his wife and kid and he got very angry. We are saying we are apologising for the whole incident. We weren't aware of any action he is taking. We are in shock like the whole country."
