(CBS) — Firefighters rescued four people trapped in a burning apartment building Tuesday morning in far west suburban DeKalb.

The fire started around 6:15 a.m. at an apartment complex at Ridge Drive and Normal Road.

When firefighters arrived, four people were trapped on the second floor. All four were rescued by ladder, and were treated at the scene, according to a DeKalb Fire Department spokesperson.

The cause of the fire was under investigation, but fire officials said it does not appear suspicious.