FORT PIERCE: The mosque that Orlando nightclub gunman Omar Mateen attended was heavily damaged in an arson fire that Muslim leaders said was the latest incident in an escalating campaign of harassment and violence against the house of worship and its members.

Given the timing ─ Sunday's 15th anniversary of 9/11 and the start of Eidul Azha ─ investigators believe the blaze that broke out shortly before midnight Sunday at the Islamic Center of Fort Pierce may have been a hate crime, St. Lucie County sheriff's spokesman Maj David Thompson said.

No one was injured. The fire burned a 10-by-10-foot hole in the roof at the back of the mosque's main building and blackened its eaves with soot.

A surveillance video from the mosque showed a man on a motorcycle approaching the building with a bottle of liquid and some papers, then leaving when there was a flash and shaking his hand as though he may have burned it, Thompson said.

The arsonist “is terrorising our community because we don't know where he is at and we don't know what he is capable of doing,” said Wilfredo Amr Ruiz, a Florida spokesman for the Council on American-Islamic Relations.

Mateen was killed by police after opening fire at the Pulse nightclub June 12 in a rampage that left 49 victims dead and 53 wounded. He professed allegiance to the militant Islamic State group. His father is among roughly 100 people who attend the mosque.

Ruiz said the mosque and its worshippers have been harassed since the massacre.

“First there were threatening voicemails,” he said. “Then drivers would splash water on the parishioners leaving on Fridays, and then a member got beat up in the parking lot when he came to the mosque for early morning prayers, and now the mosque has been set on fire.”

The mosque has received more threats since the nightclub shooting than it did in its previous 20 years of existence, assistant imam Hamaad Rahman said.

“For this to happen to us on the morning of our biggest celebration of the year has made everyone saddened and scared, but our community is bigger than a building. We are stronger than that,” he said. “I feel we will be able to pull together from this, and as time goes by we will be able to rebuild.”

The FBI and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives joined the investigation into the fire. Sheriff's officials released the video and asked for the public's help in identifying the arsonist.

Ariana Borras stopped by the mosque with her 2-year-old son to pay respects Monday morning.

Borras said she is Hispanic and not a Muslim but is sometimes asked if she is Muslim because of the colour of her skin.

The fire “makes me scared for my son. It makes me scared for my skin color,” she said. “There have been a lot of racial issues going on and there's so much hate in the world right now.”

Michael Parsons, 22, was standing outside his parents' home, directly across the street from the mosque.

He said that since the Orlando shooting, “a lot of people have been driving by hollering and yelling expletives at the church or mosque or whatever they call it.”

Parsons, a gutter installer with “trust no one” tattooed on his chest, said his mother's cancer doctor attends the mosque. “America was founded so people can believe what they want to believe and do what they want to do,” he said. “These guys flying the American flag on their trucks don't really know what the freedom is they're fighting for.”

On July 2, a man was beaten outside the mosque. CAIR said the victim was a Muslim and the attacker hurled slurs. A suspect was arrested. CAIR said that earlier that day, a man in a truck stopped outside the mosque and said, “You Muslims need to get back to your country.”

On Monday morning, a pickup truck with a “Don't Tread on Me” bumper sticker rolled past the mosque.

The driver revved his engine and raised his index finger in the air as if making the gesture for No. 1.