Firefighters were called to an apartment complex in Aurora, Colorado, on Saturday morning due to smoke in the building when an explosion occurred, injuring three people and temporarily displacing hundreds of others. 

Photos posted by Aurora Fire Rescue showed a gaping hole on the fourth floor of the building and debris on the street. 

Colorado apartment explosion

Firefighters were investigating smoke coming out of the building when the explosion took place.  (Aurora Fire Rescue)

The cause of the explosion wasn't immediately clear, but residents said smoke and strange smells preceded it. 

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"I start smelling something, start seeing a little bit of [wisps] of smoke coming up," Phillip Oransky, a resident of the building, told Fox 31 Denver. 

"I wonder what the safe distance to watch a fire is, next thing I know I just hear a bang. I see debris flying, there’s people lined against the Cheba Hut over there. First thing I can think is: not the safe distance."

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The American Red Cross is coordinating temporary shelter for 300 to 400 residents who were displaced as Aurora Fire Rescue investigates the cause of the explosion. 

Two of the injured victims were designated as "green patients," meaning they did not suffer serious injures, while third victim was transported to the Medical Center of Aurora, according to Fox 31 Denver.