A fire of unknown origin, discovered by a late afternoon cleaning crew, by 9 p.m. Thursday had destroyed the office building of Penobscot McCrum LLC at 28 Pierce St.

Shortly before 8 p.m., flames broke through the sagging ridge line of the main roof on the L-shaped building that sat on a hillside above McCrum's waterfront potato processing plant.

About 15 minutes later, flames appeared at the peak of the building's front end, facing the street. Footage posted on the internet about 9 p.m. showed the long side of the building fully engulfed. Just below are the company's potato processing buildings, which house chemicals.

Among a number of spectators at the scene was Beth Patten of Belfast who said she and a helper, Sandra Gibson, were cleaning inside the building when they realized it was on fire.

When they arrived around 5 p.m., she said, "it smelled funny, and I mentioned it to Sandra." About a half-hour later, as Patten was writing up her bill in a far office, she said, "Sandra came running down the hall saying 'This place is on fire and we need to get out!'"

"I called 911," Patten said, and they quickly fled. "As I was going by the office where it started, the light fell from the ceiling," she said, speculating that the fire might have started in the attic area above.

A fireman at the scene said firefighters got the call "about 6." Crews from Belfast, Morrill and Searsport responded. Other than the two-woman cleaning crew, the building was unoccupied.

Firefighters were able to contain the blaze to the office building, preventing it from spreading to the factory buildings below, where chemicals, including ammonia, are stored.

Belfast Fire Chief James Richards said early Friday afternoon that the state fire marshal had investigated, and the fire's origin is "undetermined." He said crews finished cleaning up the scene about 12:30 a.m Friday.

Richards estimated some 30 volunteer firefighters turned out. "We had 16 from Belfast, eight to 10 from Searsport and six to eight from Morrill," the chief said. "They all did a good job, and I want to thank them for doing that. It makes a difference."