ALBUQUERQUE, N.M. -- Two fiery explosions rocked a gasoline refinery Thursday, seriously injuring four people, officials said.

Smoke billowed from one side of the Giant Industries refinery about 15 miles east of Gallup in western New Mexico as rescue crews converged on the scene.

State police Lt. Jimmy Glascock said the explosion occurred in a part of the refinery that produces high-octane fuel, and that the blast caused a second one inside the same area.

The fire was quickly contained.

Trucker Michael Metcalf was driving nearby when the blasts occurred.

"All of a sudden, I heard a loud sound, which rocked the truck," he said. "I saw flames and black smoke coming out of the refinery, and the flames were shooting out as high as one of the towers."

Gallup Fire Chief Louie Chavez said the fire triggered the refinery's massive water guns, which helped quell the flames.

The injured were taken to hospitals in Gallup and Albuquerque. Two were listed in critical condition, a third in serious, and the fourth was in satisfactory condition.

About 50 employees were at the refinery when the explosions occurred.

The refinery was evacuated as well as a travel center about a quarter mile south, police said.

Leland Gould, a vice president of Scottsdale, Ariz.-based Giant, said late Thursday company officials were certain the first explosion was an accident, but could not confirm that until they could assess the damage.

The company planned to reopen the refinery on a limited basis today. It had been slated to shut down for a routine 30-day maintenance process.

Federal regulators and the FBI began routine investigations.