The full excavation of a mass grave of babies and young children at Tuam in County Galway is under way.

The exhumations will be carried out at the site of an institution for unmarried mothers, which operated between 1925 and 1961.

The story came to international attention 11 years ago, after amateur historian Catherine Corless discovered there were death certificates for 796 babies and children who were in the institution, but no burial records.

In 2017, investigators found what they described as “significant quantities of human remains” at the site.

The bodies were in underground chambers in a disused sewage system.

The institution, which was known as St Mary’s mother-and-baby home, was run by Catholic religious order the Bon Secours Sisters and owned by Galway County Council.

Experts from Colombia, Spain, the UK, Canada, Australia and the United States have joined Irish specialists to take part in the unprecedented excavation process.

The agency carrying out the operation is the Office of the Director of Authorised Intervention, Tuam (ODAIT), which took control of the site four weeks ago to begin preparatory work.

It is led by Daniel MacSweeney, who has described the excavation as “unique and incredibly complex”.

Scientists say the remains are largely “commingled” – in other words, the bones are mixed up.

A number of methods will be used to try to put remains back together and, where possible, identify them.

It is thought the excavation will go on until 2027, with follow-up work continuing for another three years.

About 80 people have so far come forward to give DNA samples, in the hope the bodies of their relatives may be recovered.

Anna Corrigan had two brothers who were born in the institution.

She said the start of the excavation was “both welcome and difficult”.

“While it’s a relief to see work started on the site, it’s really only the latest stage in what is still a long road for all of us,” she said.

“I won’t rest until I see justice for my two brothers who not only need a proper Christian burial but also the full rigours of the law applied.”

An entry in the institution’s “discharges” ledger says William Joseph Dolan died in 1951.

But there is no death certificate for him.

In 2013, Anna reported him to the Irish police, An Garda Síochána, as a missing person.

The following year, she also asked police to investigate what happened to John Desmond Dolan.

A certificate from 1947 registers “measles” and “congenital idiot” as the causes of his death.

Anna said: “I contended that he died of neglect and malnutrition.

“After my mother left the home, she sent the nuns five shillings a month for his upkeep.

“So how did it come to this?”

Anna’s solicitor, Kevin Winters of KRW Law, said police confirmed last month they would be issuing investigation numbers for the cases, known as PULSE records.

“It will be momentous to see the assignment of PULSE record numbers as that crystallises formal criminal investigation status upon this historical human rights debacle,” he said.

The lawyer added it was “equally important” that there were inquests into the unexplained deaths, saying the coroner should “upscale intervention after opening up the case as far back as 2017”.

Annette McKay’s mother Maggie was sent to the home in Tuam when she was 17.

She believes her older sister Mary Margaret’s remains could be in the mass grave.

“Mum’s grave still doesn’t have her name on the headstone,” she told BBC Radio Foyle’s North West Today programme.

“It has my brother’s and it has my stepfather’s and I was the one that said ‘let’s wait for Mary Margaret’.”

Hoardings have been placed around the Tuam site, and 24-hour security will be in place while the excavation is carried out, with the area preserved to the same forensic standard as a police investigation.

The Bon Secours Sisters and Galway County Council have previously apologised.

The religious order has made a contribution of £2.14m towards the cost of the excavation.