The St. Lucie County Sheriff's Office says video shows a person approach the mosque moments before flames erupted Monday.
The Florida mosque attended by Pulse Nightclub shooter Omar Mateen burned on Monday and police are calling it an act of arson.
Video captured at the Fort Pierce Islamic Center shows an individual approached the east side of the building just moments before a flash is seen and the fire starts, the St. Lucie County Sheriff's Office said in a statement.
"Evidence has revealed that this fire was set intentionally," Maj. David Thompson said in an early morning press conference held steps away from the smoldering place of worship.
In surveillance footage released by the St. Lucie County Sheriff's Office, a white or Hispanic male rides up to the mosque on a "Harley-style" motorcycle before approaching the building out of camera view just prior to when a flash of light can be seen from around the building.
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The suspect is then seen running from the building.
"[We] ask that members of the public assist us in identifying the individual who started this fire," Thomson said.
New footage taken from the scene later on Monday showed substantial fire damage to the facility.
Thompson also addressed the conspicuous time of year for such an act. It came exactly three months to the day after the massacre at the club.
In addition, Sunday marked the 15th anniversary of the 9/11 attacks as well as the start of Eid al-Adha, which is among the holiest of Muslim holidays.
"We all know the implications of the date and the time of year this is, the 9/11 anniversary. Is that related? Certainly that is in the back of our minds," he said.
Thompson went on to call the fire a horrible tragedy for the Islamic Center and said his office intends to release footage of the suspected arsonist as soon as it becomes available.
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Mateen killed 49 people when he opened fire inside an Orlando gay nightclub in June. He was shot and killed following an hours-long standoff with police.
Mateen was among the members of the Fort Pierce Islamic Center. The mosque also once counted among its members Moner Mohammad Abu Salha, an American who killed several Syrian government soldiers in a 2014 suicide bombing in Jabal al-Arbaeen after joining the Taliban.
In June, Fort Pierce Islamic Center officials told WPEC that threats of violence against the mosque had escalated.
On Monday, the St. Lucie Sheriff's Office said it was actively investigating this as an arson with the assistance of the St. Lucie County Fire District, the Florida State Fire Marshall's Office, the Bureau of Alcohol, Tobacco and Firearms and the FBI.
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