SURPRISE, Ariz. — Authorities are investigating an explosion Friday evening that injured four firefighters at an Arizona Public Service Co. facility in a Phoenix suburb.

Surprise Police Department Sgt. Tim Klarkowski said authorities responded to a report of smoke at the McMicken Energy Storage facility and called for more resources after realizing hazardous materials may have been involved.

Klarkowski said the four Peoria-Fire Medical firefighters were taken to a hospital and do not appear to have life-threatening injuries.

Four Surprise Fire-Medical Department firefighters were evaluated after the explosion.

The cause of the explosion is under investigation.