TORONTO -- At least 3,000 children, including four under the age of 10 found huddled together in frozen embrace, are now known to have died during attendance at Canada's Indian residential schools, according to new unpublished research.

While deaths have long been documented as part of the disgraced residential school system, the findings are the result of the first systematic search of government, school and other records.

"These are actual confirmed numbers," Alex Maass, research manager with the Missing Children Project, told The Canadian Press from Vancouver.

"All of them have primary documentation that indicates that there's been a death, when it occurred, what the circumstances were."

The number could rise further as more documents -- especially from government archives -- come to light.

The largest single killer, by far, was disease.

For decades starting in about 1910, tuberculosis was a consistent killer -- in part because of widespread ignorance over how diseases were spread.

"The schools were a particular breeding ground for (TB)," Maass said. "Dormitories were incubation wards."

The Spanish flu epidemic in 1918-1919 also took a devastating toll on students -- and in some cases staff. For example, in one grim three-month period, the disease killed 20 children at a residential school in Spanish, Ont., the records show.

While a statistical analysis has yet to be done, the records examined over the past few years also show children also died of malnutrition or accidents. Schools consistently burned down, killing students and staff. Drownings or exposure were another cause.

In all, about 150,000 First Nations children went through the church-run residential school system, which ran from the 1870s until the 1990s. In many cases, native kids were forced to attend under a deliberate federal policy of "civilizing" Aboriginal Peoples.

Many students were physically, mentally and sexually abused. Some committed suicide. Some died fleeing their schools.

One heart-breaking incident that drew rare media attention at the time involved the deaths of four boys -- two aged 8 and two aged 9 -- in early January 1937.

A Canadian Press report from Vanderhoof, B.C., describes how the four bodies were found frozen together in slush ice on Fraser Lake, barely a kilometre from home.

The "capless and lightly clad" boys had left an Indian school on the south end of the lake "apparently intent on trekking home to the Nautley Reserve," the article states.

A coroner's inquest later recommended "excessive corporal discipline" of students be "limited."

The records reveal the number of deaths only fell off dramatically after the 1950s, although some fatalities occurred into the 1970s.

"The question I ask myself is: Would I send my child to a private school where there were even a couple of deaths the previous year without looking at it a little bit more closely?" Maass said.

"One wouldn't expect any death rates in private residential schools."

In fact, Maass said, student deaths were so much part of the system, architectural plans for many schools included cemeteries that were laid out in advance of the building.

Maass, who has a background in archeology, said researchers had identified 50 burial sites as part of the project.

About 500 of the victims remain nameless. Documentation of their deaths was contained in Department of Indian Affairs year-end reports based on information from school principals.

The annual death reports were consistently done until 1917, when they abruptly stopped.

"It was obviously a policy not to report them," Maass said.

In the 1990s, thousands of victims sued the churches that ran the 140 schools and the Canadian government. A $1.9-billion settlement of the lawsuit in 2007 prompted an apology from Prime Minister Stephen Harper, and the creation of the Truth and Reconciliation Commission.

The research -- carried out under the auspices of the commission -- has involved combing through more than one million government and other records, including nuns' journal entries.

The longer-term goal is to make the information available at national research centre.