The director of the FBI said he would try not to use the shooter's name in a press conference updating the status of the investigation into the worst mass shooting in U.S. history.
"He made clear his affinity for ISIL and radical Islamic groups," said FBI Director James Comey, though he said the shooter was not directly connected to any terror groups overseas.
Comey told reporters Monday that Omar Mateen spoke with a 911 operator three times early Sunday morning. Mateen pledged loyalty to the head of the Islamic State group on his last call.
The FBI confirmed Mateen was interviewed twice since 2013 about inflammatory comments he had made to coworkers.
The FBI investigated Mateen for 10 months beginning in May 2013. Comey said investigators introduced him to confidential sources, followed him and reviewed details of some of his communications.
The early Sunday attack marked the deadliest mass shooting in modern U.S. history. Mateen died in a gun battle with police.
President Obama called the attack "an act of terror" and hate in an address to the nation Sunday afternoon.
The names of the victims were released Sunday night into Monday morning after families were contacted about their loved ones.
The Associated Press contributed to this report.