LINCOLN, Neb. (AP) — A police officer has been credited with alerting residents about a fire at a Lincoln apartment building.

Police say Sgt. Tom Ward spotted the nine-unit apartment building on fire early Wednesday. He radioed for help and banged on the doors of three garden-level units.

Ward was unable to reach the second and third floors so he threw rocks at windows to alert sleeping residents.

Responding firefighters used truck ladders to reach trapped residents. Officer Katie Flood says four people were injured in the fire.

Investigator Ken Hilger says a microwave inside one of the units started the fire. He estimated the fire caused $150,000 to $175,000 in damages.