Six people were hospitalized Sunday morning after a house caught fire in Littleton, possibly due to an explosion, South Metro Fire Rescue said.

The fire was reported at 10:45 a.m. in the 5900 block of South Broadway, a neighborhood near East Elementary School. Firefighters said the fire originated in the house’s basement.

Six people were injured in the fire and taken to a hospital, firefighters said. All six victims are in stable condition and expected to survive. A seventh victim was evaluated by paramedics at the scene and released.

The people inside the house told firefighters there was an explosion before the fire started. They reported that the explosion knocked out the windows of the house.

South Metro Fire Rescue is investigating the cause of the fire and the reported explosion. Firefighters said they have not confirmed the ignition source, but they have confirmed that there was a gas leak in the home prior to the fire.

Firefighters were working to ventilate the house and use thermal imaging cameras to monitor heat and smoke coming from the basement.

More information is expected to be released as the investigation continues. South Metro Fire Rescue said it has classified the fire as a “mass casualty incident” due to the number of victims treated for injuries.