ALEX, Okla. (AP) — A natural gas pipeline exploded early Friday, injuring two people, destroying three homes and forcing the closure of a highway southwest of Oklahoma City.

The explosion’s cause isn’t known.

Two mobile homes and a rock house within one-fourth of a mile of each other caught fire and were destroyed, said Gil Broyles, spokesman for the pipeline’s operator Enogex. There were no other houses in the area and no cause for further evacuation. The explosion, which could be heard for a couple of miles, caused a grass fire, Jolley said.

A man and woman in a mobile home were hurt, Broyles said.

The woman was in good condition at a medical center burn unit, a hospital spokeswoman said, with second- and third-degree burns on nearly 17 percent of her body.

A section of pipe about 16 to 18 feet long blew, causing a crater 30 to 40 feet wide, Broyles said.

The pipeline, built in 1975, transports natural gas from wells to a production plant. No problems were found when its integrity was inspected last year and when a routine maintenance operation was performed within the last week, Broyles said.