Sept. 17, 1862 is one of the bloodiest days in American history.

Mostly that’s because of the Battle of Antietam, where nearly 23,000 Americans were killed, captured or wounded, but Western Pennsylvania played a part, as well.

But at 2 p.m. that day, at the same time men were fighting and dying near Sharpsburg, Md., 78 people were killed and more than 150 wounded when the Allegheny Arsenal in Lawrenceville exploded.

While it isn’t remembered as well as the battle it shares a date with, the explosion was the worst industrial accident and largest civilian loss of life during the Civil War, according to the National Museum of Civil War Medicine.

Founded in 1814 in what was then just outside the city of Pittsburgh, the Allegheny Arsenal covered 30 acres between the Allegheny River and what’s now Penn Avenue, and was bordered on either side by what are now 39th and 40th streets.

Before the war, the arsenal had about 300 civilian workers, but records indicate by 1861 there were more than 1,100 civilian workers, many of them young women. To understand part of the reason for the need for so many workers, it’s important to understand the technology of the Civil War.

The vast majority of soldiers on both sides during the war fought with muzzle-loaded rifles, meaning they were loaded by pouring black powder down the barrel, followed by a projectile, as opposed to modern firearms that use a metallic case that holds both gunpowder and a projectile and is loaded from the rear.

But unlike Revolutionary War soldiers, who would have carried their powder in a horn or a flask and the projectiles in a pouch, many Union soldier carried paper cartridges, which were filled with black powder and a projectile.

The soldier would tear open the paper, pour the premeasured powder down the barrel and then ram the projectile home, making loading faster.

One of the major jobs at the arsenal was the creation of those paper cartridges.

The work involved handling gunpowder, making it dangerous, but it paid well and drew mainly Irish Catholic immigrants.

A National Archives blog post about the explosion and its aftermath summed up the workforce.

“Both census and payroll records show that the daughters, sisters, and wives of the immigrant families performed dangerous munitions work at the arsenal to earn desperately needed income,” it reads. “Most relied on community members such as physicians and clergymen to provide character references before they could begin work. Employment at the arsenal enticed young girls, widows, mothers, and wives who struggled to make ends meet with husbands, brothers, fathers, and sons off to war.”

By the time of the Civil War, workers at the Allegheny Arsenal were making up to 124,000 paper cartridges every day, Michael Kraus, the curator of collections for Soldiers and Sailors Memorial Hall and Museum in Pittsburgh, said in lecture about the arsenal given in 2023.

“Allegheny Arsenal is pumping out the stuff,” Kraus said.

The events of Sept. 17 would show the world exactly how dangerous working at the arsenal could be. While there is still debate about the exact cause of the explosion — or, rather, explosions, since witnesses recounted three separate explosions that leveled the building — some facts are known.

Joseph Frick, a deliveryman, told the coroner’s inquest that he saw flame along a trail of gunpowder just before the explosion.

The evidence of sometimes careless handling of gunpowder is still evident during archaeological digs today, Kraus said during his lecture on the arsenal.

In a trench dug near the site of the explosion, workers found black powder inches deep on the ground.

“This is powder that had accumulated in the labs,” Kraus said. It was supposed to be recycled, it was supposed to be taken care of, but often times it was just swept off the back porch, doused with water or just left out there. There are still remnants you can see today.”

A spark or flame set off a trail of gunpowder, which set off an initial explosion. That likely set a fire that triggered the two subsequent explosions, which happened minutes later, according to witnesses at the coroner’s inquest.

The National Archives offers a summary of why people think the explosion happened.

“It’s believed that the explosions resulted from inadequate or unenforced safety standards surrounding the handling of live gunpowder at the Arsenal,” the post reads. “Firsthand accounts reported that employees carelessly handled live gunpowder and often spilled or swept gunpowder onto the ground and into the roads extending outward from the Arsenal. The initial spark is believed to have been struck by a flinty rock on the road that led up to the door of one of the Arsenal buildings.”

There were more than 180 workers on site at the time of the explosion, and 156 of those workers were girls or women, according to the National Museum of Civil War Medicine.

The girls mostly worked in what was called the Laboratory, where paper cartridges were filled. After the explosions, a fire consumed the building, fueled by the gunpowder.

The fire burned hot, but burned itself out quickly, and witnesses were able to see the destruction wrought for themselves.

The writer for the Pittsburgh Post’s story about the explosion in the Sept. 18, 1862, edition tried to capture the gruesome nature of the scene.

“​​Of the main building nothing remained but a heap of smoking debris,” the article reads. “The ground about was strewn with fragments of dinner baskets belonging to the inmates, steel springs from the girls’ hoop skirts, cartridge paper, sheet iron, melted lead, &c. Two hundred feet from the laboratory was picked up the body of one young girl, terribly mangled; another body was seen to fly in the air and separate into two parts; an arm was thrown over the wall; a foot was picked up near the gate; a piece of skull was found a hundred yards away, and pieces of the intestines were scattered about the grounds.

“Some fled out of the ruins covered with flame, or blackened and lacerated with the effects of the explosion, and either fell and expired or lingered in agony until removed. Several were conveyed to houses in the borough and to their homes in the city. Of these, four or five subsequently died.”

There were countless tragedies that day, as families lost loved ones or saw them permanently disabled, with some of them better documented than others.

One of the best documented was that of Alexander McBride, the superintendent of the arsenal.

His daughter, Katie, worked in the Laboratory and as he rushed in her direction, he saw the roof of the building collapse with her still inside. Despite witnessing that, McBride threw himself into rescue attempts, helping to pull injured people from the debris.

In all, only 33 of the 78 killed were identifiable. Those who could be identified were given to their families for burial.

The other 45 bodies were placed in coffins and laid to rest in a mass grave in Allegheny Cemetery. The original monument, erected in 1863, deteriorated and was replaced in 1928 by the one still standing today.

It lists the names of the 45 buried at the site along with a text dedicating the monument.

“Tread softly this is consecrated dust, forty-five pure patriotic victims lie here,” it reads. “A sacrifice to freedom and civil liberty, a horrid moment of a most wicked rebellion. Patriots! These are patriots graves, friends of humble, honest toil, these were your peers. Fervent affection kindled these hearts, honest industry employed these hands, widows and orphans tears have watered this ground. Female beauty and manhood’s vigor commingle here. Identified by man known by Him who is the resurrection and the life, to be made known and loved again, when the morning cometh.”