Officials say the initial investigation shows that the fire was started accidentally. No evidence has surfaced that shows that the fire was started intentionally.
The mosque is near the intersection of West Chatham Street and Old Apex Road.
The fire started on the first floor, and workers said a window on the ground level had been broken.
"We were concerned about it...since the Pittsburgh synagogue incident, because there are a lot of copycat people," Islamic Association of Cary Presiden Shakil Ahmed said.
There was extensive damage. The fire "ate up everything except metal."
An online fundraiser has been set up to help with rebuild efforts.