Young and strapping, the 57 Irish immigrants began grueling work in the summer of 1832 on the Philadelphia and Columbia railroad. Within weeks, all were dead of cholera.

Or were they murdered?

Two skulls unearthed at a probable mass grave near Philadelphia this month showed signs of violence, including a possible bullet hole. Another pair of skulls found earlier at the woodsy site also displayed traumas, seeming to confirm the suspicions of two historians leading the archaeological dig.

"This was much more than a cholera epidemic," William Watson said.

Watson, chairman of the history department at nearby Immaculata University, and his twin brother Frank have been working for nearly a decade to unravel the 178-year-old mystery.

Anti-Irish sentiment made 19th-century America a hostile place for the workers, who lived amid wilderness in a shanty near the railroad tracks. The land is now preserved open space behind suburban homes in Malvern, about 20 miles west of Philadelphia.

The Watsons and their research team have recovered seven sets of remains since digging up the first shin bone in March 2009, following years of fruitlessly scouring the area for the men's final resting place. One victim has been tentatively identified, pending DNA tests.

The brothers have long hypothesized that many of the workers succumbed to cholera, a bacterial infection spread by contaminated water or food. The disease was rampant at the time, and had a typical mortality rate of 40 percent to 60 percent.

The other immigrants, they surmise, were killed by vigilantes because of anti-Irish prejudice, tension between affluent residents and poor transient workers, or intense fear of cholera — or a combination of all three.

Now, their theory is supported by the four recovered skulls, which indicate the men probably suffered blows to the head. At least one may have been shot, said Janet Monge, an anthropologist working on the project.

"I don't think we need to be so hesitant in coming to the conclusion now that violence was the cause of death and not cholera, although these men might have had cholera in addition," Monge said.

Other findings: Coffin nails commingled with the remains establish that at least some workers received formal burials; bones indicate the laborers were muscular despite relatively poor diets; and teeth reveal the men were not wealthy enough to afford the sugary sweets that cause cavities.

"They do have indications on their skeletons that life was not a bowl of cherries," said Monge, who is also the keeper of skeletal collections at the University of Pennsylvania Museum of Archaeology and Anthropology.

The Watsons learned in 2002 of the workers' demise from the personal papers of their late grandfather, who worked for the railroad long after the men died. Their quest, called The Duffy's Cut Project, is named for Philip Duffy, who hired the Irishmen to build a section of railroad known as a cut.

When the immigrants died in August 1832, Duffy ordered his blacksmith to burn the shanty for sanitary reasons and bury the bodies in the railroad fill, the Watsons say. The men's families were never told of their deaths.

A passenger list for the John Stamp, a ship that sailed from Ireland to Philadelphia four months earlier, offers possible identities for 15 workers who came from Donegal, Tyrone and Derry counties.

Early on, the Watsons tentatively identified one victim as 18-year-old John Ruddy, based on bone size and the ship's manifest. They have since found a section of teeth with a rare genetic anomaly — a missing upper molar that never formed — shared by some Ruddy family members in Ireland. Researchers hope for DNA confirmation in about six months.

Excavation of the burial site and the shanty, aided by ground-penetrating radar, has proved a whirlwind education in anatomy and archaeology for the 47-year-old brothers. Both earned doctorates in history but, science-wise, have nothing more than an introductory college biology class under their belts.

"It has been indeed a crash course," Frank Watson said, "and it's been fascinating."

To compensate, they have surrounded themselves with experts — including Monge and a retired coroner, forensic dentist, geophysicist and a graduate student in bioarchaeology — who share the brothers' enthusiasm and have volunteered their help.

"They're as professional a team as any one I've seen out there on a site," Monge said.

The brothers see the project as a way to document early 19th-century attitudes about industry, immigration and disease in Pennsylvania. Their ultimate goal is to recover all the remains, identify the men and inter them properly, either here or in Ireland.

Michael Collins, Ireland's ambassador to the U.S., visited Duffy's Cut last summer and said in remarks at Immaculata University that it's an important story to tell.

Immaculata has provided some funding, but the brothers are seeking grants for further DNA tests, archival research in Ireland and a Celtic cross to mark a new grave at a nearby cemetery for any remains that are not repatriated.

"We see this more as a recovery mission," William Watson said. "Get them out of this ignominious burial place."