This week marks the anniversary of an exceptionally deadly tragedy, one that still holds a sad place in the record books. It darkened the holiday season for Americans when it happened, claimed the lives of entire families, and later led to advancements in safety protection.

Yet if you live outside Chicago, you’ve probably never heard about it. This is the story of the Iroquois Theatre Fire.

The Windy City was on a roll going into the 20th century. Bold and brassy, Chicago reflected America’s growing prosperity. It was especially proud of one brand new showplace.

People were amazed when the state-of-the-art Iroquois Theatre opened on Nov. 23, 1903. The biggest stars performed on its stage. Its 1,602 seats on three levels boasted excellent views amid luxurious decor. Its location in the Loop shopping district (which street cops regularly patrolled) made patrons feel safe.

That feeling was reinforced by the theatre’s safety features. People had been skittish about going to large auditoriums ever since a fire raced through a Vienna, Austria, theatre some years before, killing nearly 400 people.

The Iroquois’ planners took that into consideration. They built 30 double-door emergency exits. Chicago’s building commissioner and fire inspector pronounced it “fireproof beyond all doubt.” (Much the way the Titanic would be declared “unsinkable” nine years later.)

Patrons raved about the new playhouse during its first five weeks of operation.

Until Wednesday, Dec. 30.

A matinee that afternoon of Mr. Blue Beard featured one of the biggest comedians of the day, Eddie Foy. (Think of him as the Will Ferrell of 1903.) With school out and people in a holiday mood, the show was sold out; some 200 people bought standing room only tickets, sitting down in clogged aisles if they found space. It’s believed as many as 2,200 people were crammed inside, a good many of them children.

To make sure nobody sneaked in, 27 of those 30 emergency exits were locked. Still, things were going so smoothly, the stage manager crept into the audience and watched the show there. Some stagehands slipped away for a drink.

Then it happened.

A light sparked a fire backstage in an area littered with oily rags and wooden scenery. Actors ran off the stage screaming. Patrons became afraid.

Eddie Foy returned to the stage and heroically tried to calm the audience, begging people to stay in their seats while a special asbestos safety screen was lowered into place. But it didn’t come all the way down. (Investigators later discovered it was made of paper.)

The lights went out, plunging the theater into darkness. A backstage door opened, letting in a draft that triggered an enormous fireball.

People fumbled blindly toward exits. But the ushers (mostly frightened teenagers) fled at the first sign of trouble without unlocking the doors. Panic turned into pandemonium. People in the balconies jumped to the floor to escape the flames.

When it was over, the scene was beyond horrible. Bodies were found stacked six deep near the locked exits.

There was a huge fallout afterward. Several people from Chicago’s mayor to the building inspector and the theatre’s owner were slapped with criminal charges. All were either acquitted or had their convictions overturned on appeal. In fact, only one person served jail time – a saloon owner convicted of robbing bodies that had been taken to his nearby establishment when it was turned into a temporary morgue.

At least 602 people were killed. By comparison, 492 perished in a 1942 nightclub fire in Boston, and 146 in New York’s infamous Triangle Shirtwaist Factory in 1911. Even the Great Chicago Fire of 1871 claimed fewer victims, an estimated 250-300.

Several safety advancements followed in the wake of the tragedy. Panic bars began to be installed in the U.S. (they’re now required in most high-occupancy buildings), and doors in public buildings began having lighted exit signs.

Not only were those safeguards too late for the 602 people who died in the Iroquois Theatre Fire, most Americans today are unaware that it even happened.

J. Mark Powell (@JMarkPowell) is a contributor to the Washington Examiner's Beltway Confidential blog. He is a former broadcast journalist and government communicator. His weekly offbeat look at our forgotten past, "Holy Cow! History," can be read at jmarkpowell.com.

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