High noon. The market’s up after a morning of light trading, and workers on their way to lunch in New York’s financial district fill the sidewalks. As the bells of Trinity Church echo through Wall Street, a horse-drawn cart stops at the corner of Wall and Broad. The driver drops the reins, jumps down, and scurries away. One minute later—at 12:01 p.m. on September 16, 1920—100 pounds of dynamite mixed with 500 pounds of scrap iron shredded the sunny streets.

The bomb killed 40, injured 300, and sent $80,000 in cash, dropped by downed bank messengers, fluttering in the breeze. Later that day, leaflets were found two blocks away calling for the release of “political prisoners … or it will be sure death for all of you.” They were signed “American Anarchist Fighters.” The “political prisoners” were probably Nicola Sacco and Bartolomeo Vanzetti, who had been indicted for murder on September 11. Investigators rounded up anarchists and questioned thousands of immigrants but never came up with a solid suspect. In 1991, historian Paul Avrich concluded that the culprit was Mario Buda, a friend of Sacco and Vanzetti’s and a fellow bomb enthusiast. Buda slipped out of the country a few weeks after the bombing, took a boat to Italy, and never returned to the U.S.

No plaque marks the site—now in the shadow of a Donald Trump skyscraper and luxury condos—but 23 Wall Street, the former headquarters of J.P. Morgan, still bears the scars: golf-ball-size pockmarks in the concrete. The bombing rattled the country and remained for 81 years the most violent attack launched on U.S. financial markets, but the next day the stock exchange doggedly marched on, closing up one point in heavy trading.