On a cold winter's day, historian Bill Watson found himself standing in the snow, picking through the roots of an upturned stump near railroad tracks in a place now known as Duffy's Cut.

The exposed roots once held in their grip buttons, human bones and old coffin nails - vital clues in a centuries-old unsolved mystery.

The stump, pulled up several years ago, stood over the final resting place of seven of 57 Irish laborers who perished at the railroad construction site in 1832, during an outbreak of cholera. Also found at the scene was a skull that had been pierced by a bullet and cleaved by a hatchet.

"It's not just cholera," said Watson, who with his twin brother and fellow historian, Frank Watson, is leading the excavation project to piece together what may turn out to be a grisly tale of anti-immigration violence from the 1800s.

For the last 10 years, the Watsons and their research team have struggled to find out what happened to the crew toiling under a boss named Philip Duffy, as they cut a swath through the heavily wooded terrain to lay train tracks about 32 km northwest of Philadelphia.

The brothers' interest in the site began in 2002, when they discovered references to the immigrant laborers in a document file compiled nearly a century ago by Pennsylvania Railroad president Martin Clement and later kept by his personal assistant - the Watsons' grandfather.

Those documents indicated that all 57 laborers, hired right off the boat from Ireland, died of cholera within six weeks of arrival. The number was far more than the eight deaths listed in local news accounts at the time.

While the cause appeared to be cholera, physical evidence uncovered at the scene also hinted at cruelty and murder, the Watson brothers said.

"We have no idea what percentage of these guys were murdered," said Bill Watson, who chairs the history department at nearby Immaculata University. "But if we have 57, it's the worst mass murder in Pennsylvania history."

Ground-penetrating radar found what researchers believe is the gravesite on land now owned by Amtrak. Excavating there could yield vital clues.

But the project has stalled while awaiting permission from the rail company to dig near its tracks. The team has been quietly relying on a network of political backers to press Amtrak for the go-ahead. Negotiations are ongoing, but the historians and the railroad remain hopeful.

"We are optimistic that a plan can be devised allowing safe access to the location for the research team to continue their project while not interfering with railroad operations or compromising anyone's safety," Amtrak said in a statement.

The team began digging in 2004, unearthing tobacco pipe shards and old forks. Those findings alone raised suspicions because, the Watsons said, poor laborers would not have discarded such useful, valuable items.

Then, in 2009, the team found a human tibia, or shin bone.

In all, they found the remains of seven people - all near the old poplar tree - three exhibiting signs of having met a violent end. Exactly who killed them is not known.