Mateen was killed by law enforcement officials after fatally shooting 49 people and wounding 53 others in a gay nightclub in Orlando in June.
Local law enforcement officers received reports of flames rising from the Islamic Center of Fort Pierce, located about 100 miles (161 km) southeast of Orlando, at about 12:30 a.m. EDT, St. Lucie County Sheriff’s Major David Thompson told reporters at a news conference. No one was injured.
The attack occurred on one of the holiest Muslim holidays.
Surveillance video showed a person approach the mosque moments before the blaze erupted, he said.
“Immediately after the individual approached, a flash occurred and the individual fled the area,” Thompson said.
Investigators will work to enhance the footage to identify the suspect, he said.
Thompson said investigators were still seeking a motive for the attack and were considering a connection with the 15th anniversary of the Sept. 11 attacks on Sunday.
“I would not want to speculate, but certainly that is in the back of our minds,” he said.
The Muslim holiday Eid al-Adha, a major Muslim holiday, is being celebrated on Monday and also could have prompted the attack, Thompson said.
The mosque temporarily relocated its morning prayers for Eid al-Adha, also known as the Feast of Sacrifice.