A series of explosions heard miles away ripped through a weapons and ammunition factory Friday, setting the plant ablaze and killing four people, fire officials said.

The initial explosion was reported around 11:30 a.m. at the Buffalo Rock Shooters Supply company in Ottawa Township, just oustide the Naplate city limits, about 75 miles southwest of Chicago. The first blast was heard 4 miles away, Naplate Fire Chief Vince Dettore said.The explosions and fire destroyed all three buildings that housed the company's gunshop and warehouse, where "millions of rounds" of ammunition were manufactured, stored and sold. Dettore said the company produced an array of ammunition, including bullets, black powder and "anything in arms." A shooting range also was on the property.

Four bodies - believed to be those of a man and three women - were pulled from the rubble between 3:30 and 5:30 p.m., Dettore said, adding they were killed instantly. The bodies were taken to the LaSalle County coroner's office and had not been identified. Dettore said they were believed to be company employees.

A few nearby homes were evacuated as a precaution.

The cause of the explosions and fire were not known, Dettore said. The state fire marshal's office and federal Bureau of Alcohol, Tobacco & Firearms joined the investigation.

"We will be going back tomorrow to determine the cause," Dettore said Friday night.

Forty firefighters fought the blaze, hampered by a series of explosions that ripped the buildings, Dettore said.

It was third fire at the company since the 1970s. The main building was destroyed by one of those blazes in the 1970s.