The mosque where Pulse nightclub shooter Omar Mateen occasionally worshiped was intentionally set on fire, authorities in Florida said.

The St. Lucie County Sheriff’s Office said emergency crews responded to the Islamic Center of Fort Pierce shortly after midnight Sunday and extinguished the flames that were spilling out of the mosque.

Authorities are investigating the incident as arson.

“A fire at any place of worship is alarming, regardless of the circumstances,” the sheriff’s office said in a statement. “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.”

Sheriff’s officials said they will soon release the video to the public and ask for help identifying the suspect in the arson case.

The fire came at the beginning of Eid al-Adha – the holiday of the sacrifice. The Islamic Center of Fort Pierce had an Eid prayer scheduled for 9 a.m. Monday. Those who wanted to worship were instructed to go to another mosque in the area.

“It is with a very heavy heart that we have to announce that last night around midnight, there was an arson attack on our Mosque,” the Islamic Center said in a statement early Monday. “Please keep us in your Du’as and prayers.”

Early Monday, Maj. David Thompson with the St. Lucie County Sheriff’s Office told reporters it was a “horrible tragedy not only for the Islamic center but for our community.”

“I don’t want to speculate on a motive,” he said. “We all know the implications of the date and the time of year that this is – the 9/11 anniversary. Is that related? I wouldn’t want to speculate, but certainly that is in the back of our minds.”

Thompson told reporters that “evidence has revealed that this fire was set intentionally.”

The mosque became a locus of attention in June, after Mateen opened fire at a gay nightclub in Orlando, killing 49 people and leaving dozens of others injured. Mateen was killed in a shootout with police inside the Pulse nightclub.

Mateen occasionally attended the Islamic Center of Fort Pierce, coming to prayers with his father and young son. Mateen’s three sisters were active volunteers at the mosque, which has about 150 congregants.

Mateen was at the mosque just two days before the deadly shooting rampage.

The first American to carry out a suicide bombing in Syria, Moner Mohammad Abusalha, also occasionally worshiped here before he left his car outside the mosque in 2014, flew to Syria, burned his U.S. passport and blew himself up in an operation for an al-Qaida affiliate.

Ibrahim Hooper, national spokesman for the Council on American-Islamic Relations, said attacks against the American Muslim community are becoming a daily occurrence in America since Islamophobia moved into the mainstream.

“Unfortunately, within the past year, we’ve seen an unprecedented rise in bigotry in our society,” he said Monday, adding: “It’s becoming a great concern to the American Muslim community.”