Federal authorities say a duplex explosion that killed at least four people on the Pine Ridge Indian Reservation in South Dakota has been deemed an accident.

The Bureau of Alcohol, Tobacco, Firearms and Explosives said Saturday that a certified fire investigator has found the blast was likely caused by a propane line leak and an accidental spark.

Oglala Sioux Tribe spokesman Kevin Yellow Bird Steele has said the explosion happened Thursday in the town of Pine Ridge and that several injured people were taken to hospitals.

He didn't identify the victims. Yellow Bird Steele has said tenants in the duplex smelled gas and were searching for a leak before the explosion.

Federal officials say more information will be made available in the coming days.