HUMBOLDT, S.D. (AP) — One man was killed in a house explosion Wednesday morning in rural South Dakota and two other occupants were so seriously hurt that they were flown to a hospital, authorities said.

Capt. Josh Phillips, of the Minnehaha County Sheriff’s Office, said a bystander who spotted the house engulfed in flames reported it around 7 a.m. and helped the victims.

Several volunteer fire crews from the surrounding area rushed to the home, which is located just a few miles south of the town of Humboldt. The Sioux Falls Fire Department also responded, but the home was leveled.

“This just wasn’t a house fire and a minor explosion. This was a significant event,” Phillips said, adding that the state fire marshal is working with investigators from the sheriff’s office and other agencies to determine the cause.

The name of the man who was killed in the blast wasn’t immediately released. Phillips declined to provide details on the man and woman who survived.