Firefighters have determined that a weekend fire at an apartment building was caused by a heater possibly coming in contact with towels hanging in a bathroom. Firefighters were dispatched to a fire at an apartment building in the 3700 block of Karstens Drive shortly after 11 a.m. Saturday following a call reporting smoke coming from the second floor of the building, according to an incident report. First responders arrived at the apartment around 11:08 a.m. and found the apartment filled with thick black smoke. Crews quickly found and extinguished the fire before working to ventilate the building. Officials said no residents were home at the time of the fire. A subsequent investigation revealed that a heater in a bathroom likely caught fire when it came into contact with towels nearby.