ABBOTSFORD, B.C. -- Indigenous leaders from across British Columbia and parts of the United States gathered at a sacred burial site in Abbotsford, B.C., alongside the developer who owns it, calling for its protection by the provincial government.

From a grassy plateau overlooking farmland in the Fraser Valley, Sema:th First Nation Chief Dalton Silver told those who assembled on Friday that they were standing on a mass grave.

Hundreds, if not thousands, of Sema:th and Sto:lo First Nations were killed by smallpox carried by colonists, wiping out an estimated 80 to 90 per cent of their populations, he said.

The Sema:th have been fighting for years to have the 65-hectare property known as Lightning Rock returned to them.

"I can't really explain in detail the cultural and spiritual importance for our people to protect the resting places of our ancestors," Silver said.

John Glazema of Cold Water Ranch Developments said he was among a group of developers who purchased the site in 2011 with plans to build an agricultural mall but only learned of its spiritual and cultural significance a year later.

"So we stopped. And we did the right thing, we're going to respect our neighbours and we're going to do what's right with respect to these burial grounds. So we're not going to dig it up," he said.

Since then, he said they have been in negotiations with the province to return the site to the Sema:th people, but have yet to reach a settlement for their $12 million in costs.

The lengthy process and large interest payments have been costly for the developers and their families, Glazema said.

They were hopeful two years ago after signing a letter of understanding with the province that said they would receive reimbursement for exactly what they had paid.

"And still we're here fighting for our money back," he said.

More than a dozen Indigenous leaders, including former lieutenant-governor Steven Point and Grand Chief Stewart Phillip of the Union of B.C. Indian Chiefs, signed a joint letter to Premier John Horgan calling for the lands to be returned.

Indigenous Relations Minister Scott Fraser was unavailable to comment but his office issued a joint statement with the Ministry of Forests, Lands, Natural Resource Operations and Rural Development.

"We acknowledge that Lightning Rock is a sacred place for Sema:th. We have had extensive discussions with Sema:th and the landowner, and we continue to work to find a solution to protect the Lightning Rock site," the statement says.

The site isn't protected under the specific criteria of the Heritage Conservation Act, so the ministries say they're working to find "an alternative solution."

"We have respected the request of the nation that the site not be disturbed. That has meant we have been unable to obtain the archeological evidence required for the protection under the Heritage Conservation Act," the statement says.

The province remains interested in finding a solution, it says.

Dave Schaepe, an archeologist who has worked with the Sto:lo for 20 years, said the process shows there's a two-tiered system for protecting sites in Canada, where there's an "intangible" connection between human lives and the land.

He gave the example of the Slesse Mountain airplane crash monument near Chilliwack, B.C., which is recognized and protected legally as a memorial to the 62 people who died the 1956 crash but whose bodies were never recovered.

"There's an aspect of a similar situation, intangible heritage where there's a combination of humans and people with the land. That's protected but this area is not," he said.

In addition to the burial grounds, Silver explained that Lightning Rock is also an important cultural site. According to oral history, an overconfident shaman came into conflict with the powerful creature Thunderbird, who turned him to rock and split him in four pieces.

All major anthropological and archeological bodies in the world have endorsed a declaration on the safeguarding of Indigenous ancestral burial grounds as sacred sites and cultural landscapes, Schaepe said.

"There's a basis here and a call to action more broadly to do something about situations like this and the Lightning Rock exemplifies this need," he said.