DeKalb Fire Department officials still are investigating the cause of the Grant Towers fire that caused hundreds of students to evacuate Sunday morning.

The fire occurred at 8:05 a.m. Sunday on the 13th floor of Grant North’s “C” tower.

The follow-up investigation found the cause to be “undetermined,” according to a DeKalb Fire Department press release.

However, investigators have “left the incident open” and will continue to work with the State Fire Marshal’s office on the investigation.

Investigators were unable to find the exact cause of the fire because it was located in a “structural void area” on the 13th floor of the tower, according to the release.

The only person injured was a firefighter who suffered a minor back strain while advancing a hose to help fight the fire. Department officials refused to release his name, but said he is doing well.

The floor contained mechanical equipment and one student apartment, the resident of which reported the fire.

Upon arriving at the floor, firefighters were met with considerable fire and visible smoke, according to reports. However, early assessments indicated minimal damage, and clean-up focused mainly on water removal.

Lt. Greg Hoyle, also a DeKalb fire investigator, said Sunday that the fire burned at the stairwell’s air exchanger and at the exterior wall of the residence hall.

University Police reported that the origin of the fire appeared to be a small heater or heat register in a stairway outside the 13th floor, according to an NIU press release. The stairwells are self-contained and insulated from the rest of the building, so little, if any, smoke escaped into living areas.

Firefighters removed roof panels to extinguish the fire by spraying water through the walls.