ALEX, Okla. (AP) - A natural gas pipeline exploded early Friday in Grady County, injuring two people, destroying three homes and forcing the closure of a highway.

A 20-inch round line exploded less than 50 yards north of State Highway 19, two miles east of Alex. The highway between Alex and Bradley could be closed until noon, a state Transportation Department spokeswoman said.

"The immediate threat is over. Everyone has been accounted for," Grady County sheriff's Capt. Robert Jolley said.

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 man's identity and the extent of his injuries isn't known. He was taken to Grady Memorial Hospital in Chickasha. Mildred Hull was in good condition at the Integris Baptist Medical Center burn unit, a hospital spokeswoman said. She has second- and third-degree burns on nearly 17 percent of her body.

Two people in a mobile home that caught fire weren't hurt, Broyles said. The rock house was unoccupied.

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

"When the pressure drop was noted, we manually closed valves on either end of the leak," Broyles said.

The explosion's cause isn't known. Enogex and Oklahoma Corporation Commission inspectors have been sent to the scene.

Grady County Fire Chief Perry Wenzel, who lives 20 miles from the explosion site, said he could see flames from his house.

"They (the victims) are very lucky to be alive," Wenzel said.

The pipeline, built in 1975, transports natural gas from wells northwest of Thomas in Custer County to a production plant south of Alex.

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.