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.
Mateen told police in a 911 call that he had pledged his allegiance to the head of the Islamic State militant group, though investigators do not believe he had any help from outside organizations.
Shortly after the massacre, the mosque in Fort Pierce was identified as Mateen’s place of worship. It has reported receiving multiple threats of violence and intimidation. In June a motorcycle gang circled the center and shouted at its members, and in July a Muslim man was beaten outside the mosque.
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.
(Reporting by Laila Kearney in New York; Editing by Scott Malone and Jeffrey Benkoe)