Federal regulators and the FBI are investigating two explosions that rocked a New Mexico gasoline refinery April 8, The Associated Press reported.

The blast, seen by trucker Michael Metcalf, who was driving nearby when the explosions occurred, seriously injured four people, officials said.

“All of a sudden, I heard a loud sound, which rocked the truck,” Metcalf 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.”

After the explosions, 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 contained.

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 and two were satisfactory.

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.