"This was an act of violence, born out of hate, that inflicted terror on an entire community," said Ron Hopper, special FBI agent in charge of the case.
"So I would call it a hate crime. I would call it terrorism. It's both," Xinhua news agency quoted Hopper as saying.
At least 49 people were killed and 53 others were wounded, including a police officer, on Sunday in a shooting spree at a popular LGBT nightclub in Orlando, Florida.
It was the worst mass shooting in US history and the deadliest terror attack since the 9/11 in 2001.
Meanwhile, according to US TV network NBC News, Mateen's wife had reportedly feared that Mateen was planning to attack a gay nightclub before Sunday's massacre and pleaded with him not to commit violence, but she did not inform the police.