Joseph Schreiber, 32, was accused of setting fire to the Islamic Center of Fort Pierce on the evening of September 11, 2016. 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 crews responded 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.," according to a statement released at the time by 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."
Schreiber, a Floridian, was identified and arrested in connection with the fire
two days later. He was charged with arson of a structure evidencing prejudice.
Gosnell said Schreiber's plea of no contest Monday was adjudicated as a guilty plea.
had attended the Islamic Center of Fort Pierce since 2003, Imam Syed Shafeeq Rahman told CNN last year. Mateen usually came for Friday prayers before the rampage at the Orlando nightclub
on June 12.
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 said 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."