Mateen was killed by law enforcement officials after killing 49 people and wounding 53 others in a gay nightclub in Orlando in June.
Law enforcement officers received reports of flames rising from the Islamic Center of Fort Pierce, located about 100 miles (160 km) southeast of Orlando, at about 12:30 a.m. EDT (0630 GMT), St. Lucie County Sheriff's Major David Thompson told reporters at a news conference. No one was hurt.