ELWIN, Ill. (AP) _ A gas pipeline exploded this morning, sending fireballs into the sky and killing at least one person. Four others were injured, one seriously.

The blast killed one of 30 people who were working on the pipeline when the explosion occurred around 10 a.m., said Capt. John Wrigley of the Macon County Sheriff's Department.

The pipeline was being moved to make way for highway expansion, said John Barnett, a spokesman for its owner, Panhandle Eastern Pipe Line Co.

''I could see a spray of gas going straight up and going into the wind,'' said Rhonda Morrell, who was driving on nearby highway U.S. 51 at the time of the explosion. ''I thought it was going to blow. I turned around and headed back. Just as I did, I saw it shoot up.''

The explosion blew a truck off the road, seriously injuring a 64-year-old man in it, said Lewis Mahoney, spokesman for St. Mary's Hospital in Decatur. Three other people, two of them firefighters, were treated for less serious injuries, he said.

WCIA-TV of Champaign said two fireballs estimated at 100 feet high could be seen from the station's camera mounted on the sixth story of a building in Decatur, about eight miles from the scene.

Intense flames and blistering heat made it difficult for rescue workers to reach the scene, Wrigley said.

State police Sgt. Richard Kaelin said the flames had died down by 11:30 a.m., but there were still a few minor grass fires near the highway, which passes within about 200 feet of the blast site.

U.S. 51 was closed indefinitely.

Barnett, speaking from Eastern Panhandle headquarters in Houston, said company workers had uncovered about 100 feet of the pipe and had moved about 25 feet away when it ruptured. The cause of the accident was unknown, he said.