There is no clear evidence that the Orlando shooter was directed by the so-called Islamic State group (IS), US President Barack Obama has said.
The inquiry into Sunday morning's attack, in which 49 people were killed, was being treated as a terrorist investigation, he said.
US authorities say gunman Omar Mateen pledged allegiance to IS shortly before the attack.
However, the extent of his links to IS remains unclear.
Speaking in Washington, the president said: "It does appear that at the last minute he [gunman Omar Mateen] announced allegiance to Isil [IS].
"But there is no evidence so far that he was in fact directed.
"This is certainly an example of the kind of home-grown extremism that all of us have been concerned about for a very long time," Mr Obama said.
The deadliest mass shooting in recent US history ended when police shot Mateen dead. The attack also left 53 people injured.
Orlando Regional Medical Centre said on Monday that a number of the victims being treated there remained critically ill, and five patients were in a grave condition.
Earlier on Monday, Florida officials gave details of the attack at the Pulse nightclub early on Sunday.
An off-duty officer working at the club had initially fought Mateen in a gun battle. Shortly after, more police officers arrived.
They engaged Mateen, forcing him to retreat to the toilet, where he was holding hostages, Orlando police chief John Mina said.
Police were able to rescue "dozens and dozens" of people from the nightclub, some of them injured, once the gunman had retreated to the toilet, he said.
Mateen phoned the police from the toilet, Mr Mina said, and made a pledge of allegiance to the so-called Islamic State group (IS) while speaking to them.
Chief Mina said that statements made by the suspect while he was holed up in the toilet, and information from people trapped inside, had convinced police that further loss of life was imminent.
Orlando Mayor Buddy Dyer said the shooter and some hostages were in one toilet, and another 15 or so people were in another, across the hallway.
After officers broke through the exterior wall, hostages came out through the opening, the mayor said.
Mateen himself followed them out shooting and was killed, he added.
