Police are investigating a fire at the mosque that was attended by the Orlando nightclub gunman who killed 49 people.
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.
The mosque blaze was so severe that it burned a hole 10ft-by-10ft (3-by-3m) in the roof of the mosque's main room.
Someone was filmed "just moments before a flash is seen and the fire starts," theSt Lucie County Sheriff's Office said on Facebook.
"A fire at any place of worship is alarming, regardless of the circumstances," it added.
A spokesman for the sheriff's office, Major David Thompson, said: "This is a horrible tragedy. Not only for the Islamic Center, but for our community".
The arsonist "is terrorising our community because we don't know where he is at and we don't know what he is capable of doing", said Wilfredo Amr Ruiz, a Florida spokesman for the Council on American-Islamic Relations.
"For this to happen to us on the morning of our biggest celebration of the year has made everyone saddened and scared, but our community is bigger than a building," Mr Amr Ruiz said.
"We are stronger than that."
Multiple law enforcement agencies are investigating, including the FBI.
The Islamic Center of Fort Pierce has experienced other trouble in recent months. A few weeks after the nightclub shooting, a man was badly beaten outside the mosque.
Motorists frequently stop to shout abuse at worshippers, including the driver of a truck who stopped to hurl insults earlier that same day, according to the Council on American-Islamic Relations.
In addition to Omar Mateen, the mosque was also visited by Moner Mohammad Abu Salha, who became the first American to commit a suicide bombing attack in Syria.