Hundreds of children who died at a Scottish orphanage run by the Daughters of Charity of St. Vincent de Paul were allegedly buried in a mass grave, according to a new investigative report.

The Smyllum Park orphanage was operated in Lanarkshire, Scotland, from 1864 until it closed in 1980.

A joint investigation by The Sunday Post and the BBC claims up to 400 children who died at the institution in its 116-year existence were buried in a mass grave at St. Mary’s Cemetery in Lanark, located about a mile from the orphanage. The cemetery also holds the remains of the sisters and other staff members who died at the institution, although their graves are marked with headstones.

A memorial with the words ‘Sweet Jesus have mercy on the souls of the children of Smyllum’ is the only marker over the grave of the children.

According to The Sunday Post, one adult – 21-year-old Louise Langlois – is also buried in the mass grave. A one-time resident, she later served as a servant at the facility until dying of pneumonia in 1945.

The religious order had previously said it only knew of 120 children who died during the period, but after sifting through decades of death certificates, the two media organizations determined the actual number was closer to 400: Averaging one child’s death every three months.

The report said most of the children died of natural causes; and diseases such as tuberculosis, pneumonia and pleurisy all took their toll. A third of those who died were under the age of 5.

Sometimes, the death rate at the orphanage was triple that in the general population, although this is not uncommon in care facilities, where disease can spread quickly.

The orphanage is currently undergoing investigation by the Scottish Child Abuse Inquiry, after previous residents accused staff of neglect and physical abuse.

The government inquiry was established in 2015 to investigate 60 care facilities in Scotland, with a final report and policy recommendations due in 2019.

Testifying before the inquiry in June, Sister Ellen Flynn – the head of the British province of the Daughters of Charity of St. Vincent de Paul – said there was no evidence the accusations were true.

“The first view is that we are extremely saddened that accusations were made,” Flynn said. “We are very apologetic, but in our records we can find no evidence or anything that substantiates the allegations.”

After the report was published on Sunday, the order refused to comment on the specific allegation that 400 children were buried in the mass grave, saying the official inquiry “is the most appropriate forum for such investigations.”

The statement also said, “As Daughters of Charity our values are totally against any form of abuse and thus, we offer our most sincere and heartfelt apology to anyone who suffered any form of abuse whilst in our care.”

In response to the latest allegations, a spokesman for the Catholic Church in Scotland told the newspaper The National, “Any suggestion that the deaths of some children were caused by anything other than natural causes should be investigated to the fullest extent possible.”

The statement also said the Catholic Church never had any responsibility for or ability to place children in care – “that has always been and remains a matter for the statutory authorities who placed children in care and were subsequently responsible for their welfare. These authorities alone can comment on the outcome of some of their placements and the standard of care expected.”

Beginning in November, the government inquiry will begin a new phase with a “particular focus” on the Smyllum Park orphanage and the Bellevue House children’s home in Rutherglen, which was also run by the Daughters of Charity.

Earlier this year, between 700 and 800 children were discovered buried in unmarked graves at a home for unwed mothers run by the Bon Secours Sisters in Tuam, Ireland. The children died in a period dating from 1925 to 1961.