In the early afternoon on September 17, 1862, just about 200 miles from where the Battle of Antietam was taking place, another Civil War-era tragedy occurs: Three explosions rip through the Allegheny Arsenal and kill 78 workers, mostly young women and girls. At 2 p.m., the first explosion hits, followed shortly by two more; then, an intense fire tears through the building known as the Laboratory, where many people were making cartridges.

The tragic Allegheny Arsenal explosion was the worst civilian disaster during the Civil War, and it eerily happened on the same day as Antietam—making September 17 the bloodiest single day in American history, indeed. The Allegheny Arsenal —located in Lawrenceville, a neighborhood northeast of downtown Pittsburgh along the Allegheny River—was a critical supply outlet for the Union Army. Originally built in 1814, workers at the 38-acre complex manufactured gunpowder, cartridges, harnesses and other equipment needed for the war that began in 1861.

On that tragic late-summer day, 186 workers were on site at the Allegheny Arsenal, and 156 of them were women and young girls; at the time, there were no child labor laws. The girls and women replaced many males who worked there and were repeatedly fired for bringing smoking supplies like matches to work, which was extremely hazardous at an arsenal. With lax safety standards, workers often carelessly handled live gunpowder, and often spilled it onto the floor and the road leading to the arsenal.

Wagoner Joseph R. Frick was making many arsenal deliveries that day, and just before the explosion, he turned onto the macadamized road that led to the Laboratory. After Frick unloaded several 100-pound barrels of gunfire, he noticed spilled gunpowder burning on the road, but it was too late to stop the explosions. While investigators never reached an official conclusion about the cause of the mysterious explosion and theories abounded, many agree that the most likely explanation was a tiny spark from a wagon wheel or horseshoe on the rough road igniting the spilled gunpowder.

Rev. Richard Lea, pastor of nearby Lawrenceville Presbyterian Church, ran across the street after the explosion destroyed his church’s windows. He jumped the arsenal wall to provide aid and comfort to the dying. Many years later, in a sermon dedicated to the explosion victims, Lea said: “But amidst all this dismay and fearful consternation and apprehension of still worse to come, when the magazine should explode, there were many who entered the gates and climbed the walls, determined to aid, or die in the attempt.”

The Daily Ohio Statesman reported this about the explosion: “The scene was appalling, dead bodies lying in heaps as they had fallen.”

The shattered remains of 43 of the Allegheny Arsenal explosion victims were buried at Allegheny Cemetery, located just down the road in Lawrenceville. A marble memorial at the gravesite was dedicated in 1928, after the original one from 1863 weathered away over time. The dedication on the memorial calls the explosion “a horrid memento of a most wicked rebellion.”