PORTAGE, Ind. (AP) _ An explosion ripped apart a section of a 5-year-old steel mill today, killing at least three people and injuring eight others.

``All of a sudden there was a huge explosion, like a time bomb. Everything went kaboom,'' said Lee Spitka, a supervisor who was at the mill.

The explosion happened in the southwest corner of the Beta Steel mill, said Bill Fritchley, director of planning and development for the Indiana Port Commission.

The blast blew sheets of metal siding off the three-story building, leaving a gaping hole. It also partially collapsed part of the roof of a Beta office complex about 50 feet away, Fritchley said from Indianapolis.

Spitka said a tank in the basement had exploded and about 15 people were nearby. The tank held water or hydraulic fluid under high pressure, he said.

``People were just running all over the place. They were scattering because they didn't know what was going to fall next,'' Spitka said.

``It was dark and scary when the explosion happened. It was terrifying,'' said employee David Varney, who also was inside the mill. He appeared uninjured.

Three people were killed and eight were injured, according to city police Sgt. John Rogers. He said authorities believed everyone in the plant was accounted for.

About 50 to 60 employees were inside the mill at the time of the 7 a.m. blast, Rogers said. Because of a shift change, most workers were not on the production floor at the time, he said at a news conference at the site.

``Inside looked like a city of ruins,'' Rogers said.

The hole in the roof allowed sunlight to stream into the otherwise darkened corner of the building hours later.

State occupational safety inspectors and investigators from the federal Bureau of Alcohol, Tobacco and Firearms were on the scene, Rogers said.

``We're looking at several different things. We just don't know right now,'' Rogers said when asked about possible causes.

Porter Memorial had treated four people and none appeared to have life-threatening injuries, said spokeswoman Sandra Parker.

Two people were treated for broken bones at St. Mary's Hospital in nearby Hobart, said spokeswoman Sally Jensen. One was in serious condition and the other in fair condition, she said.

Beta Steel, which began operations in 1991, is located in the southwest corner of the 500-acre port on Lake Michigan. It's the largest of the 30 companies operating there, with about 250 employees, Fritchley said. The rest of the port was still operating this morning.

Portage is about 30 miles southeast of Chicago.