Police say an overnight fire at the Islamic Center of Fort Pierce, the Florida mosque that Pulse nightclub shooter Omar Mateen attended, was an act of arson.
Surveillance footage shows a person approaching the mosque "just moments before a flash is seen and the fire starts," the St. Lucie County Sheriff's Office says on Facebook.
"A fire at any place of worship is always a tragedy," Maj. David Thompson said in a video posted by the sheriff's office. "Evidence has revealed that this fire was set intentionally."
Police responded to reports of a fire at the mosque in Fort Pierce, Fla., around 12:30 a.m. ET, and firefighters put out the fire. The building was unoccupied at the time of the fire, Thompson said.
The person seen on the surveillance footage fled the scene, investigators say. Authorities are working on releasing the footage and are requesting the public's help in identifying the suspect.
Multiple federal agencies are assisting in the investigation.
Thompson said police don't want to speculate about motive, but the fact that the fire began the night after the 15th anniversary of the Sept. 11, 2001, attacks is "in the back of our minds."
The fire was also set during Eid al-Adha, a major Muslim holiday. Police say members of the Fort Pierce mosque will be moving their observances elsewhere to allow the police investigation to proceed.
"This is a horrible tragedy not only for the Islamic Center but for our whole community," Thompson said.
Fort Pierce is located on the east coast of Florida, about 100 miles from Orlando. Mateen, who lived in Fort Pierce, perpetrated the deadliest mass shooting in modern U.S. history in June. He killed 49 people at Pulse, a gay nightclub in Orlando, pledging allegiance to ISIS over the phone before being killed by police.
The overnight arson is not the first time the Fort Pierce mosque has seen trouble since the Pulse massacre, as The Associated Press reports:
"A few weeks after the nightclub shooting, a man was beaten outside the mosque, according to Sheriff Ken J. Mascara. Early on July 2, deputies were called by a man who said someone was trying to burglarize a vehicle. Arriving deputies found a man bleeding from the mouth who told them he was approached by someone who 'asked him what he was doing and then punched him several times in the face and head.'
"The man was stopped by deputies and arrested a short time later.
The Council on American-Islamic Relations said in a statement that a man in a truck had stopped outside the mosque earlier that day and made slurs, including, 'you Muslims need to get back to your country.' "