The information was gleaned from close scrutiny of about 5,000 death records selected from an initial screening of 250,000 records going back to the 19th century.
"It's staggering to think that families would not have known what happened to a child that was sent off to the residential schools," Chief Coroner Dr. Andrew McCallum told The Canadian Press.
"There was a huge vacuum of information. What was fed back to the immediate family was highly inconsistent."
At the beginning of the year, Ontario's coroner's office began trying to identify missing and dead children from the residential schools at the request of the national Truth and Reconciliation Commission.
Among other things, the commission's "Missing Children Project" has been trying to come to grips with the large number of aboriginal children who died or went missing while in the care of the scores of government-funded, church-run residential schools, the last of which shut down in the 1990s.
In all, about 150,000 aboriginal children were forced to leave their communities for the schools in an effort to assimilate them into mainstream Canadian society. By some estimates — especially prior to the 1940s — mortality rates reached 50 per cent.
Although some deaths were suicides, most fatalities were due either to disease or occurred after the children ran away from the schools, then had accidents, hypothermia or drowned.
"They were terribly unhappy and they left," McCallum said.
"They succumbed to various things that happen to children who are on their own in harsh environments."
McCallum, who noted the residential school deaths still resonate, also announced a joint inquest into the deaths of seven aboriginal students in Thunder Bay, Ont., between 2000 and 2011.
The seven — aged 15 to 21 — died after leaving their home communities to pursue secondary education in Thunder Bay.
The joint inquest was called after consultation with Nishnawbe Aski Nation due to the similar circumstances which surrounded the deaths, McCallum said.
The Ontario review has so far turned up 120 possibly "missing" children, although cross-referencing with commission information still needs to take place to confirm that, said Dr. David Eden, who led the records search on behalf of the coroner's office.
Eden called the overall mortality numbers linked to the schools "scary" and said the entire system beggared belief.
"I can't understand why this was done and how it was done," Eden said in an interview.
"I can understand the need for both First Nations and the larger population of Ontario and Canada to acknowledge that it happened and we should do everything we can to tell the affected families what happened to their child."
The records search — aimed at complementing information already in possession of the commission — has been a painstaking process, particularly for those from before 1965 now kept in the Archives of Ontario.
In addition, records related to the Indian residential schools were often poorly maintained.
McCallum said his office would share its approach with other chief coroners and medical examiners next week at a conference in Quebec City in hopes of helping their searches.
Despite the difficulties, McCallum said it was essential to dig up the information.
"It's really important for there to be a truthful, as much as possible, disclosure to those who were involved as to what happened," he said.
The Truth and Reconciliation Commission was set up in 2007 as part of the federal government's apology for the residential school system.