The cause of a four-alarm fire last summer at an apartment building in Plymouth will remain a mystery after the state fire marshal’s office concluded its investigation this week, the Plymouth fire chief said Thursday.

Nobody was hurt in the July 12 fire at the Vicksburg Village Apartments, which had 20 of its 62 units severely damaged. Fifty to 60 people were forced to flee the building on Rockford Road, Fire Chief Rick Kline said in a news release.

The investigation identified several ignition possibilities but was unable to isolate a primary source, so it officially ruled the fire of undetermined cause.