On July 24, 1915, a Great Lakes tour boat called the Eastland rolled over while docked on the Chicago River near Clark Street, killing 844 people. It was the worst disaster in the city’s history. (Second deadliest? The July 1995 heat wave.) Michael McCarthy, 52, who wrote about the wreck in the 2014 book Ashes Under Water: The SS Eastland and the Shipwreck That Shook America, explains what it means a century later.

What drew you to this tale?

We basically had a Titanic-sized disaster in the center of the continent, and I was stunned that we still didn’t really know how it happened. The fact that no one was found criminally responsible for the deaths of 844 people is one of the great injustices of the 20th century. I think there was a cover-up.

You discovered that the Eastland’s owners tried to blame a whistleblower, Joseph Erickson. Who was he?

The chief engineer—the second most powerful person after the captain. He was basically in charge of everything on the ship that moved. It was easy to blame him. But the problem wasn’t him; it was the machinery he was supposed to operate. The Eastland had nearly capsized twice before, and in each case it had to do with this hydraulic stabilizing machinery in the belly of the ship. It mishandled very easily and made the ship toss about. Erickson told the police chief the day after the Eastland sank that the owners had spoken to him about making repairs, but they decided to postpone them.

So how did the trial end without a conviction?

Because of the way the charges were structured. The court would have to find all six of the defendants [including Erickson and the ship owners] guilty or all six of them innocent. So famed attorney Clarence Darrow—who represented the ship officers, owners, and inspectors—took the case to make sure the owners didn’t roll over on Erickson. He did some very creative and shrewd work in the courtroom. Darrow was doing the right thing by getting the innocent guy off, but in so doing, he necessarily had to get off the people who were responsible.

How did the disaster change Chicago?

In Chicago and elsewhere, there are thousands of descendants of the victims. Given that 22 whole families were erased in a single day, it really changed the architecture of family trees.

What’s the takeaway?

Calamity can result when corporations put profit in front of people’s safety. We’re seeing this in the auto industry right now—all kinds of revelations about problems in cars. If safety is not paramount when machinery transports people, the cost in human life can be devastating.