Carla Pusateri, a fire prevention officer for the Corvallis fire department, said the fire at the Salman Al-Farisi Islamic Centre was started early in the morning. She said "quite a bit of evidence" was left at the scene which led her to believe the fire was intentionally started.
In an FBI sting operation on Friday, Mohamed Osman Mohamud parked what he thought was a bomb-laden van near Portland's Christmas tree lighting ceremony and went to a nearby train station, where he dialled a number on a mobile phone that he believed would detonate the vehicle. Instead, federal authorities moved in and arrested him.
The case is the latest in a string of alleged terror plots by US citizens or residents, including one in which a man pleaded guilty earlier this year to trying to set off a car bomb in New York.
Yosof Wanly, imam at the Oregon centre, said Mohamud was a normal student who went to athletics events and drank the occasional beer.