A house explosion in a northern suburb of Milwaukee on Saturday injured six people, including a firefighter and a police officer, fire officials said.The cause of the blast was not immediately clear, but is under investigation, said Kerry Wenzel, a spokeswoman with the North Shore Fire Department.Video from CNN affiliate WISN showed a gaping hole where a house once stood, as firefighters doused the debris with water.Fire Chief Robert Whitaker said three structures were involved in the incident -- the house that was destroyed and those on either side of it.Among the injured were two residents, a neighbor and a passerby, both of whom tried to assist those inside, he said.The injured police officer was treated for smoke inhalation, while the firefighter was treated for heat exhaustion, the chief said.