A fire broke out at the Islamic Center of Fort Pierce in Florida shortly after midnight on Monday morning, evidently serious enough for flames to be seen from outside the building. This was the mosque attended by Omar Mateen, the mass murderer who attacked the Pulse nightclub in Orlando.

The authorities quickly determined that the fire was intentionally set. The Orlando Sentinel quotes St. Lucie County Sheriff’s Major David Thompson describing surveillance video that showed someone approaching the east side of the building.

“Immediately after the individual approached, a flash occurred and the individual fled the area,” Thompson said.

He told reporters his department has asked the FBI and BATF to assist in the investigation, and said technicians were currently working on cleaning up the surveillance video, so it could be released to the public.

The Orlando Sentinel reports that although “leaders at the Islamic Center have said that since the nightclub shooting the facility has received threats and that security had been stepped up,” Thompson said his department had “received no recent threats,” and described conditions in the area leading up to the 9/11 anniversary as “peaceful.”

“A fire at any place of worship is alarming, regardless of the circumstances,” he added. “This is a terrible tragedy, not only for the Islamic Center, but for our community.”

The Palm Beach Post quotes Thompson saying it was too early to connect the fire at the Islamic Center to Mateen or 9/11. “You know, that’s always a conclusion that someone could jump to. As I said, at this time we’re unsure even the cause of the fire is, so I wouldn’t want to speculate whether or not this was related or unrelated,” he said.

In addition to Sunday’s 9/11 anniversary, Monday marks the beginning of the Muslim holiday of Eid al-Adha. The Islamic Center issued a statement urging people to attend a different mosque for Eid prayers.

NBC News in Miami reports that the Islamic Center was unoccupied at the time of the fire, and the extent of damage to the building is not yet clear. WPTV in West Palm Beach flew a news chopper over the Islamic Center, and reported seeing “substantial damage.”