US historians trying to uncover a mystery surrounding the mass death of 57 Ulster immigrants, nearly 180 years ago, have uncovered evidence they may have been murdered.

Previously it had been thought the group — who died within weeks of starting gruelling work on the Philadelphia and Columbia railroad in 1832 — were cholera victims.

However four skulls unearthed from the grave suggest the men suffered blows to the head and at least one may have been shot in an out-pouring of anti-Irish violence.

Dr William Watson, chairman of the history department at Immaculata University and his twin brother, Frank, have spent the past eight years trying to unravel the mystery surrounding the deaths of the Irish workers at Malvern, Pennsylvania.

Dr Watson said the revelation that at least four of the men had died violent deaths proves “this was much more than a cholera epidemic”. Anti-Irish feelings ran high in 19th Century America.

The men lived in a shanty near the railway tracks where they worked.

It is now believed that while many died of cholera, some were killed by vigilantes because of prejudice, tension between affluent residents and the poor transient workers, or because of a fear that the cholera would spread.

“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,” anthropologist Janet Monge, who is also involved with the project, said.

Close examinations have revealed a number of clues about the men's lifestyle. Their bones indicate that while they had poor diets, the labourers were still muscular.

Coffin nails were also recovered from the site indicating some were given a formal burial.

However, it is not thought the families in Ireland were told what happened to their loved ones.

Using passenger records, the Watsons believe some of the group had sailed from Ireland to Philadelphia as little as four months before their deaths.

It is thought they were originally from counties Donegal, Londonderry and Tyrone.

The brothers hope to eventually recover all the remains, identify the men and give them a proper burial, either in the US or in Ireland.

“We see this more as a recovery mission — get them out of this ignominious burial place,” said Dr Watson.