Investigators say an arsonist may have been the cause of an apartment fire Wednesday evening that damaged multiple units. Firefighters were called to the scene at the Villa Norte Apartments on Fullerton St. around 5:20 p.m. after officials say an off-duty firefighter saw the door of an apartment open and flames inside. He closed the door to stop the fire from spreading while crews rushed to the scene. Officials say the flames appeared to start in the downstairs apartment before spreading to the unit above it. Both apartments suffered fire and smoke damage. The two units opposite also suffered light smoke damage. It took 25 firefighters to bring the blaze under control. Witnesses told authorities they saw someone enter the apartment, and shortly after they left, the apartment caught fire. The cause of the fire remains under investigation.