DUBLIN -- A mass grave containing the remains of babies and young children has been discovered at a former Catholic orphanage in Ireland, government-appointed investigators announced Friday in a finding that offered the first conclusive proof following a historian’s efforts to trace the fates of nearly 800 children who perished there.

The judge-led Mother and Baby Homes Commission said excavations since November at the site of the former Bon Secours Mother and Baby Home in Tuam, County Galway, had found an underground structure divided into 20 chambers containing “significant quantities of human remains.”

The commission said DNA analysis of selected remains confirmed the ages of the dead ranged from 35 weeks to 3 years old and were buried chiefly in the 1950s, when the overcrowded facility was one of more than a dozen in Ireland offering shelter to orphans, unwed mothers and their children. The Tuam home closed in 1961.

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The history of unwed mothers forced into hard labor is a well-documented dark chapter in Irish history.

For nearly four decades, thousands of these “fallen women” toiled at the place in Tuam, called simply, “the Home,” to atone for having out-of-wedlock children.

While many of the women eventually left “the Home” after serving their time, many of their children apparently were not so fortunate.

Friday’s findings provided the first proof after decades of suspicions that the vast majority of children who died at the home had been interred on the site in unmarked graves. That was a common, but ill-documented practice at such Catholic-run facilities amid high child mortality rates in early 20th century Ireland.

The government in 2014 formed the investigation after a local Tuam historian, Catherine Corless, tracked down death certificates for nearly 800 children who had died as residents of the facility - but could find a burial record for only one child.

“Everything pointed to this area being a mass grave,” said Corless, who recalled how local boys playing in the field had reported seeing a pile of bones in a hidden underground chamber there in the mid-1970s.

The government’s commissioner for children, Katherine Zappone, said Friday’s findings were “sad and disturbing.” She pledged that the children’s descendants would be consulted on providing proper burials and other memorials.

“We will honor their memory and make sure that we take the right actions now to treat their remains appropriately,” Zappone said.

This picture shows a shrine in Tuam, County Galway on June 9, 2014, erected in memory of up to 800 children who were buried at the site of the former home for unmarried mothers run by nuns. Getty

The report found that the dead children may have been placed in underground chambers originally used to hold sewage. Corless said she found records stating that the sewage systems were used until 1937, when the home was connected to a modern water supply.

A decommissioned septic tank had been “filled with rubble and debris and then covered with top soil” and did not appear to contain remains, the report said. But excavators found children’s remains inside a neighboring connected structure that may have been used to contain sewage or waste water.

The commission’s finding that most of the remains date to the 1950s corroborates Corless’ collection of death certificates. It also dispels a popular argument that bones seen at the site might predate the orphanage’s opening, when the building was a workhouse for the adult poor, or even be from people who died in the mid-19th century Great Famine.

Labour Party lawmaker Joan Burton said the Tuam orphanage’s dead may have been interred “without normal funeral rights, and maybe even without their wider families having been made aware.” She called on the Catholic Church to provide more assistance to investigators.

The investigators, who are examining the treatment of children at a long-closed network of 14 Mother and Baby Homes, said they still were trying to identify “who was responsible for the disposal of human remains in this way.”

The Bon Secours Sisters order of nuns, which ran the home until its closure, said in a statement that all its records, including of potential burials, had been handed to state authorities in 1961. It pledged to cooperate with the continuing investigation.

Corless criticized the Bon Secours response as “the usual maddening nonsense. They must apologize and take responsibility for what happened there.”

She called on the nuns to promise explicitly to help the state organize proper marked burial places for every dead child once each set of remains could be identified.

“That’s the least that can be done for them at this late stage,” she said.