ROCKFORD, Ill. (WTVO) — Three Rockford firefighters were injured in a four-hour campaign against a blaze that broke out at a multi-unit self-storage facility early Tuesday.

Officials said the fire spread from one unit to many others at Central Avenue Mini Warehouse & Containers, located at 2151 N. Central Avenue, around 3:35 a.m.

The Rockford Fire Department said the fire spread to adjacent units before it could be extinguished. Officials said firefighters forced the doors of 40 units to stop the fire’s advance, and the contents of approximately 12 units were damaged by flames, the others by smoke.

A firefighter was taken to the hospital for burns. Another was taken to the hospital for exhaustion. Another was treated at the scene.

A fire investigator is working to determine the cause of the fire, officials said.

The fire caused an estimated $300,000 in damages.

Fires at self-storage units happen, but they are rare. Michael Schnaper, arson investigator for the Rockford Fire Department, said that neatness counts when preventing fires in places where there is a lot of stuff, like garages. That means that residents should not overpack.

He also said that they should consider what they are putting inside.

“I would advise people, one, avoid your flammable liquids and things like that and avoid anything that could be a source of ignition, any heat source, things like that,” Schnaper said. “But, on top of that, make sure whatever you are storing in there is properly stored. It’s safely stored, you’re not putting something that’s combustible or ignitable near something that could be a possible heat source, and you’re not stacking things to the point where you can’t get firefighters into the room.”