Over several decades in the late 1700s and early 1800s, at least 70 enslaved African Americans labored under arduous and hazardous conditions at a forge and foundry in Frederick County that produced, among many other items, ammunition used in the Revolutionary War.

Many lived out their lives at the Catoctin Furnace, located near the small bucolic town of Thurmont, a few miles from where the presidential retreat Camp David would be built decades later. As ironworkers, they were industrial slaves distinct from their agricultural counterparts who worked on plantations picking cotton or sowing rice. The ironworkers ran every step of making iron implements, from mining the ore to chopping the wood that fueled the intense fires that transformed the ore into usable goods.

At least 32 enslaved people, whose labor created great wealth and prosperity for others, were buried in relative anonymity next to the ore pits. Over the years, they disappeared into the dirt and trees.

“They were forgotten,” said archaeologist Elizabeth Comer, president of the Catoctin Furnace Historical Society. “The narrative at Catoctin became the narrative of the European workers who moved in.”

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In 1979, a construction crew discovered their remains while excavating ground to expand the nearby U.S. Route 15, the Catoctin Mountain Highway. Today, the highway would almost certainly be rerouted, but more than 40 years ago, the decision was made to excavate the remains so construction could resume as planned. With advancements in DNA technology, Comer began an exhaustive re-analysis in 2014 of those remains, which had been put in the care of the Smithsonian Institution.

This week, the journal “Science” published research that connected 41,799 living people to 27 of the workers who were buried at the ironworks — the largest concentration of living relatives were found in Maryland, suggesting many descendants of the workers remained in the area.

Comer, who is an author of the published study, called it a first step. The next step, she said, is to “ethically contact” living descendants. There might be some, she said, who do not want to know.

“It’s not our data,” Comer said. “It belongs to the descendant community. We’re just the vehicle.”

According to the study, most of the 41,799 reside in the U.S., and the vast majority are distant relatives who simply share ancestors. About 3,000, however, might be direct descendants, potentially the great-great-great-grandchildren of the Catoctin ironworkers.

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They and the other relatives, whose identities are not known to anyone, were identified from a pool of 9.3 million consenting participants who submitted DNA samples to the biotech and genomics company 23andMe, whose researchers conducted the study using the David Reich Lab at Harvard University. Among the study’s other authors are Reich, 23andMe geneticists Éadaoin Harney and Steven Micheletti, Douglas Owsley of the Smithsonian Institution, and Henry Louis Gates Jr., Harvard historian and the host of the popular genealogy TV show “Finding Your Roots.”

“For the Catoctin individuals and many other enslaved African Americans, most of the information that we can gather from historical documents treats them as property, rather than people with unique and meaningful lives,” Harney said by email. “We hope that this study helps to rectify this by rediscovering a part of their story.”

Excavation at Catoctin revealed 35 graves. Three were empty; the skeletal remains of 32 were found. Of those, 29 were viable for possible DNA extraction. Comer explained that for the best results, researchers needed skulls, in particular jawbones. From 29 jawbones, the genomes of 27 people were sequenced.

Five families of mothers, children and siblings were identified among the 27 workers, who were found to be related to people not just in this region but in nearly every part of the country, and in parts of west and central Africa, as well as Great Britain and Ireland.

All but one of the 27 workers buried at Catoctin Furnace had primarily African ancestry; the highest rates of genetic sharing were with people who identified as members of the Wolof and Mandinka ethnic groups, from present-day Senegal, Mali, and the Gambia, and the Kongo ethnic group of the Democratic Republic of Congo. Of the workers with European ancestry, genetic markers were strongest in the Y sex chromosomes, passed from men to male children — this is evidence of the sexual exploitation of African American women by male European-American slaveholders, according to the study.

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According to the Catoctin Furnace Historical Society, the cemetery is believed to be the most complete African American cemetery connected with early industry in the U.S. Enslaved people were buried there in what appears to have been unceremonious fashion.

They were not laid to rest in tranquil areas set apart from the village, but rather “right in the middle of everything,” Comer said. “It was always an unquiet place.”

Comer theorizes the foundry owners buried enslaved people there intentionally so they could keep an eye on mourners.

The workforce at Catoctin included masons, carpenters and colliers who turned wood into the charcoal that powered the furnaces. Workers there produced the shells fired by George Washington’s army in the Battle of Yorktown, as well as household items such as pots, utensils, and stoves.

The foundry was built near large deposits of iron ore. The ironworks was an important part of the country’s early economy and, like all labor-intensive industries of the time, relied on enslaved people. The work was extremely physically demanding and exposed workers to toxic pollutants. Sometime in the 1840s, the furnace owner began employing paid labor, mostly European immigrants. The operation grew and kept producing iron until the turn of the next century. By then, the role played by enslaved Americans at the foundry had been largely forgotten.

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“Recovering African American individuals’ direct genetic connections to ancestors heretofore buried in the slave past is a giant leap forward both scientifically and genealogically, opening new possibilities for those passionate about the search for their own family roots,” Gates Jr. said in a statement by Harvard Medical School.

Harney said her company is “considering the best way to thoughtfully and ethically return results to our customers. Since this is the first time it has been possible for us to identify direct genetic connections that link historical individuals to their living relatives, we want to make sure that any results we return are communicated as clearly as possible and in a way that minimizes any potential harm.”

There is already a promising lead in that process. By researching documents and records, Comer identified two present-day families, the Patterson family and the Summers family, which both have ancestors who worked at the foundry. It is possible some of them were buried there. Family members have spread out into the country, but many remain in or near Maryland.

The Catoctin study is one of several underway around the country to attempt to fill gaps in our history. Enslaved African Americans were not listed in the census by name until 1870, about the time many started using surnames for the first time. Many of them changed surnames more than once. While exhuming bones from African American burial grounds is a sensitive issue, the efforts have been met mostly with support from local communities and possible descendants.

The results of the study, its authors write in its conclusion, “serve as a model for obtaining direct insights into the genome-wide genetic ancestry of enslaved people in the historical US.”