CHICAGO (CBS) — Chicago was marking the 100th anniversary of the deadliest disaster in the city’s history on Friday.

On July 24, 1915, the SS Eastland, a steamship preparing for an excursion to Michigan City, Indiana, capsized in the Chicago River near LaSalle Street, with about 2,500 people on board.

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More than 840 people were killed, including 22 families, when they became trapped after the ship toppled on its side.

“Everyone wanted to go out on the Eastland because it was the first ship scheduled to go out that day,” author Jay Bonansinga told CBS 2’s Vince Gerasole. “It was such a freak accident nobody expected this giant 300 foot steel ship to go over.

‘When it went over, it was instant death for many people because they were below decks.”

The ship already was top-heavy.

And, with 2,500 passengers, many more than should have been on board, moving around the vessel, the boat tipped over, trapping hundreds of people under the water line.

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In the immediate moments, nearby welders ran to the ship, to cut holes in the hull to reach the living and the dead. A morgue was set up in the West Loop’s armory where families walked by hundreds of bodies to identify their loved ones.

On Friday, descendants of those who died gathered to remember the tragedy, near the site where the ship capsized.

“It was extremely emotional this morning when I was here at 7:30, at the exact time when the ship had capsized 100 years later,” said Karl Sup, founder of the Eastland Memorial Society. “Really knowing what my grandparents went through, and collecting, and hearing all the stories from other descendants of Eastland survivors and victims, knowing the stories, it was extremely emotional.”

Some 290 children died, and 70 percent of the victims were between 15 and 30 years old. The Eastland was one of five boats hired to take employees of the Western Electric company and their families to Michigan City.

Nancy Jeffers-Aden came all the way from California for Friday’s memorial ceremony. Her great grandfather was Martin Jeffers, a bridge tender credited with saving at least 15 people by jumping into the river after the Eastland capsized and helping pull people to safety.

Supporters of the Eastland Memorial Society have been trying to build a permanent memorial at the site of the disaster; including statues, and a large black umbrella bearing the names of those who died.

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Historians said it rained for seven consecutive days after the disaster, so the city was dotted with black umbrellas for a week, emblematic of an entire city’s heartbreak.