CHICAGO (WBBM NEWSRADIO) — The Chicago Fire Department Wednesday marked the 50th anniversary of an explosion that killed four firefighters in the Back of the Yards neighborhood responding to a fire in a sausage factory.

On Feb. 7, 1968, more than 300 firefighters were called to the Mickelberry Food Products Company at 50th and Halsted.

Fire Cmsr. Jose Santiago, addressing those assembled, including firefighters and family members of some of those who lost their lives, said, “We remember the day when the gates of hell opened up, and the lives of so many were changed forever.”

He said hearing the radio traffic from that day will send chills up one’s spine.

A gasoline truck making a delivery in the alley behind the plant exploded after a valve hit a garbage dumpster, spilling gas into the basement. Following a series of explosions, firefighters attempted to rescue people who had gone to the roof.

A massive explosion then leveled part of the building and threw the firefighters off their ladders.

Family members of the firefighters who died that day laid a wreath at the scene.

Five civilians, including the company’s president, who ran back inside, were also killed.

More than 70 others were injured.