Law enforcement officials are investigating the fire at the Florida Mosque, where Orlando gunman Omar Mateen prayed, as an act of arson.
A mosque where Orlando nightclub shooter Omar Mateen worshipped was targeted in an arson attack, US authorities in Florida said on Monday, without ruling out the possibility of a hate crime.
A 911 call at 12:31 am local time (0431 GMT) reported flames coming out of the Islamic Center of Fort Pierce, Major David Thompson of the St Lucie County Sheriff's Office said.
No one was injured during the incident and the extent of the damage was not immediately clear.
"We are not sure exactly whether this is a hate crime or just an arson, we do know that a crime has occurred and that it is arson," he said.
"We do not have evidence right now that says it's a hate crime. Of course, because this is a place of worship and because this incident occurred after the anniversary of 9/11, we're going to explore that."
Photographs posted on the sheriff department's Facebook page showed fire engines parked in front of the building, which is a former church, with the facade appearing undamaged.
Authorities were working on enhancing the security video to help identify the perpetrator.
The FBI and other federal agencies are collaborating with the local sheriff's office in the investigation.
Investigators, still seeking a motive, were considering a connection with the 15th anniversary of the Sept. 11 attacks on Sunday, Thompson said.
"For this to happen to us, on the morning of our biggest celebration of the year, was something horrific," Hamaad Rahman, associate imam at the mosque, said at a news conference.
"Our community is bigger than a building, we are stronger than that," he added. "Hopefully as time goes by, we will be able to rebuild."
It urged the faithful to attend another mosque in the city to participate in prayers on Eid ul-Adha, a Muslim holiday marking the end of the annual Hajj pilgrimage.
The mosque in Fort Pierce, identified as Mateen's place of worship, has reported 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.
Forty-nine people were killed and another 53 injured on June 12 when Mateen, a 29-year-old US citizen, opened fire on the Pulse gay nightclub in Orlando.
Mateen told police in a 911 call that he had pledged allegiance to DAESH, though investigators do not believe he had help from outside organizations.
Mateen was killed by police.