The fire broke out early Monday morning at the Islamic Center of Fort Pierce. Mateen, who killed 49 people at the Orlando nightclub in June, had attended the mosque.
No one was inside when the fire started, authorities said. St. Lucie County responded to the blaze around 12:31 a.m., after two drivers called 911 to report flames on the building's roof.
"Surveillance footage from cameras on the building revealed that a white or Hispanic male arrived on motorcycle at approximately 11:38 p.m.," said a statement from the St. Lucie County Sheriff's Office. "In the video, it appears he is carrying paper and a bottle of some type of liquid. The male approaches the northeast side of the building and a flash is seen, presumably when the fire ignited. The male is then seen running from the building."
Police are now seeking a white or Hispanic male, wearing a button-down shirt, jeans with embroidery on the back pocket and a boonie or bush hat. The man may have burned his arm while setting the fire and he is driving a Harley-Davidson-style motorcycle, possibly with saddlebags, police said.
Firefighters extinguished the blaze. Photos released by the sheriff's office showed the structure appeared to be intact.
Authorities were trying to make sure the building was safe before going inside to assess the extent of the damage, Thompson said.
It's not clear exactly what the arsonist used to set the building on fire. The flames were extinguished by 5 a.m., authorities say.
Hamaad Rahman, the son of the mosque's imam, appeared at a news conference with members of Florida's Muslim community. While a fire at any house of worship would be a concern, they said, this event was especially worrying given the 9/11 anniversary, which fell this year on the day before the Muslim holiday of Eid al-Adha
Worshipers were at the mosque until roughly an hour before the fire was reported, and many area Muslims had to find alternative plans Monday for Eid al-Adha because of the extensive damage to the center.
Inside the mosque
Mateen had attended the Fort Pierce Islamic Center since 2003, Imam Syed Shafeeq Rahman told CNN earlier this year. Mateen had usually come for Friday prayers before the rampage at the Orlando nightclub on June 12 that killed 49 people.
Rahman said the mosque preached a peaceful message.
"So far as we here in Fort Pierce are concerned, we are always very conscious -- consciously speaking about peace, about love, about unity," Rahman told CNN in a previous interview. "We came to this country; this country gave us everything."
The mosque had also been attended by Moner Mohammad Abu-Salha,
a Florida man who became the first known American suicide bomber in Syria.
One of the mosque's attendees told CNN that the Fort Pierce Islamic Center was "like any other mosque."
"It's simple, friendly," Bedar Bakht told CNN in June. "Our imam is very good at explaining things. We never hear any stupid things coming out from anybody. It's very low-key. So for two kids [Mateen and Salha] to be growing up like this, it's very, very strange for me."