TORONTO -- WARNING: Details in this story may be distressing to some

Forensic experts say identifying the remains of Indigenous children and adults buried at former residential schools is a painstaking process that could take decades.

On Thursday, Cowessess First Nation says it found 751 unmarked graves at the site of the former Marieval Indian Residential School in Saskatchewan.

The find comes less than a month after the remains of 215 Indigenous children were found buried in unmarked graves near the former Kamloops Residential School in Kamloops, B.C.

Working with experts from Saskatchewan Polytechnic, community members scoured the area around Marieval earlier this month – the ground penetrating radar detecting so many anomalies, the initial search had to be restricted.

Phase one of the search focused on the 44,000 square metres of a former gravesite on the grounds, with phase two set to expand beyond that to search for remains that may have been buried without a casket. 

The process of identifying victims of the forced assimilation at the residential schools requires specialized expertise and technology, like drones with terrain mapping abilities, along with the First Nation oral history of the trauma and memories of the survivors and their descendants.

Megan Bassendale, who is the director at Forensic Guardians International, said the process of identifying all the remains will require extensive investigations and needs to be led by the communities themselves.

“There's a lot of information that needs to be collected from a lot of different sources to actually be able to make identifications,” Bassendale told CTV News Channel on Thursday. “It can take decades.”

During the identification process, forensic investigators would need to interview the family members of missing people, residential school survivors who attended the same school or any other contacts, in an attempt to find out if they had any identifying features.

“They may have had a broken leg during their life or a chipped tooth. Anything that can individualize one person from another is really important for the forensic identification process,” Bassendale explained. “You have to talk to families and communities and listen to them and listen to their stories, there will be a lot of probably oral history.”

Dean Hildebrand, a DNA advisor who also works for Forensic Guardians International, echoed Bassendale on CTV News Channel Thursday, saying the process of identifying the remains will be “extremely challenging.”

“Generally speaking it’s a two-part process, you need to deal with the unidentified human remains themselves and trying to get DNA from those samples,” he explained. “That’s half the battle because you have to have something to compare that to - typically you would have a personal effect or a family member.”

Hildebrand said in the case of the graves at Marieval, finding comparative samples is “very challenging,” and he expects that investigators will be “combing through records for leads” on who these families could be.

So far documents aiding the search effort include a burial plot map post-1960, a cemetery survey dated 1887 to 1952, and local death records spanning 40 years after 1952 – but a lot of information is missing. 

Generally speaking, Hildebrand said “a standard unidentified human remains case” doesn’t take long to process DNA samples, “you can get results in a matter of days.”

“But in this case of course it’s very complex in terms of the scale and scope of what’s been found,” he said. “There’s a lot of work that has to be dealt with -- the sheer volume and numbers of the individuals involved – and also the length of time we’re dealing with, these individuals have been buried for an extended period of time and that is a complex element.”

The Tk'emlúps te Secwépemc First Nations have already begun the work to identify the 215 children whose bodies were found at the Kamloops site, although their attempts to obtain some of the relevant records has been met with resistance from the Catholic groups that ran the school.

While this is a slow and arduous process, time is of the essence as Indigenous elders, residential school survivors and others who may remember key details continue to age.

“If somebody’s critically ill and they’re the only person that knew somebody that was in one of these schools, then it’s really important for them to record that,” she said.

Bassendale noted that the process is quite “invasive” given the level of personal information that is required.

“It's very invasive in terms of personal information because we need very specific details to actually be able to identify human remains. And, you know, some people aren't comfortable with that. Some people are.”

For these reasons, Bassendale pointed out that some families or communities may not want to put themselves through the traumatic process of exhumation and identification, and added it’s important to respect those wishes if that’s the case.

Vice-Chief Heather Bear of the Federation of Sovereign Indigenous Nations and member of Ochapowace First Nation in Saskatchewan, who went to the residential school, told CTV National News that while “in Western medicine or Western science there are ways of identifying remains,” when it comes to using DNA in the Marieval search effort, “it’s too early to comment.”

“Some families in some communities may decide to memorialize sites and leave it at that. But if the process of exhumation and identification is chosen, then it's important that families understand that it's a long process,” said Bassendale.

Bassendale says it's critical for Indigenous communities to lead this process. Otherwise, bodies could end up becoming misidentified.

“If they don't believe or trust the information that's coming out of the process, then it just doesn't work,” she said.

With files from CTV News’ Brooke Taylor

The Indian Residential School Survivors Society toll free line is: 1 (800) 721-0066.

A 24-hour crisis line for residential school survivors is: 1 (866) 925-4419 if you require further emotional support or assistance.