The story of 57 Irish immigrants who died while building Pennsylvania's first railroad in the 1830s has taken a potentially sinister and the site of a mass grave in which they were buried may now be a murder scene.

A team of volunteer investigators trying to piece together what happened to the men believe at least two of them may have been killed by local vigilantes after attempting to escape a cholera outbreak at the area known as Duffy's Cut, about 30 miles west of Philadelphia.

The remains of the men were discovered at the site of a mass grave there after local historians began an archae­ological dig on foot of a six-year investigation.

To date, they have recovered four sets of remains from five graves, two of which included skulls which have evidence of "time of death" blunt force trauma.

This indicates these men may have been killed after an outbreak of cholera in the valley forced them to try to flee the area, researchers believe.

William Watson, professor of history at Philadelphia's Immaculata University, his brother Rev Frank Watson, and a team of academics and researchers are attempting to trace what happened to the group of 57 Irish immigrant railroad workers.

The men arrived in the US on 23 June 1832 but died around eight weeks later during a cholera epidemic which affected hundreds of people in the Philadelphia area. They had sailed from Derry in April 1832 and came from Donegal, Tyrone, Derry and Leitrim.

"When cholera hit their camp in August, the men tried to seek help in the community, but they were forced back into the valley to die. Usually, 40% to 60% of cholera victims died, but in this case, all 57 died," Dr William Watson told the Sunday Tribune.

"We have long suspected that there was more at play here than just cholera. There were already reports of deteriorating relations between one of Duffy's earlier crews of railroad workers and locals only a mile east of the site of Duffy's Cut."

"There are authenticated reports of cholera victims being killed out of fear of contamination in nearby Chester, Pennsylvania, during the 1832 cholera epidemic, and also of people who harboured cholera victims there being killed."

Now, recent discoveries at the site appear to have confirmed their worst fears.

"We have five graves and four sets of remains excavated (two of which include cran­iums). Our forensic team has found evidence of blunt force trauma on the first two craniums, with probably perimortem [ie 'time of death'] injuries," Watson said.

"The first man located was the last one buried – whom we believe is 18-year-old John Ruddy, who came from Donegal. His skull and teeth indicate his age.

"We have enough parts of his face that we can do a facial reconstruction and the teeth will allow us to obtain his DNA to locate living descendants in Ireland," he added.

"The railroad file on the event reports that the men were forced back into the valley, and the only agency that could have done that was the East Whiteland Horse Company, the local vigilante organisation."

Those working on the project hope eventually to rebury the men properly –the first five graves excavated to date included coffins, but investigators do not expect to find coffins for the remainder of the men thought to be buried nearby.

With an average age of just 20, most of the men buried there brought no possessions with them to the US.

The area where they worked was called Duffy's Cut after the contractor, Philip Duffy, who was himself an Irish immigrant.