Surveillance cameras show a suspect approaching the Islamic Center of Fort Pierce moments before Monday morning's blaze, fire officials said.
The fire may have been timed to coincide with the Muslim holiday of Eid al-Adha and the anniversary of 9/11.
Omar Mateen committed the worst mass shooting in modern American history in June when he opened fire in Pulse.
He was killed by police after taking hostages and declaring his allegiance to the Islamic State group.
