PINE RIDGE, S.D. — A duplex explosion that killed at least four people on the Pine Ridge Indian Reservation in South Dakota has been deemed an accident, federal authorities said Saturday.

The Bureau of Alcohol, Tobacco, Firearms and Explosives said in a statement that a certified fire investigator has found the blast was likely caused by a propane line leak and an accidental spark. The statement said more information will be made available in the coming days.

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 in Nebraska and South Dakota.

Yellow Bird Steele didn’t identify the victims, including the dead, whom he described as three females and one male. He has said tenants in the duplex smelled propane gas and were searching for a leak before the explosion.

Estelline Black Feather, 66, awoke from a nap on her couch Thursday afternoon when her house shook, she told the Rapid City Journal. Believing her propane tank had blown, she went outside and saw that a nearby duplex had been leveled.

“People were running,” she told the newspaper. “There was a real bad smell, like sulfur.”