US historians trying to uncover a mystery surrounding the mass death of 57 Irish immigrants nearly 180 years ago have uncovered evidence they may have been murdered.
Previously it had been thought that the group which died within weeks of starting gruelling work on the Philadelphia and Columbia railway in 1832 were cholera victims.
Two skulls unearthed at a probable mass grave near Philadelphia last week showed signs of violence, including a possible bullet hole. Another pair of skulls found earlier at the 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.
Dr 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 mystery.
Anti-Irish sentiment made 19th-century America a hostile place for the workers, who lived amid wilderness in a shanty near the railway 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 hypothesised 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 up to 60%.
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 sustained blows to the head. At least one may have been shot, said Janet Monge, an anthropologist working on the project.