The Toronto Zoo says it's investigating the deaths of three wood bison.

The zoo says in a release that two females and one male were found dead Friday morning in a holding area that is not accessible to the public.

"Staff are saddened by this rare and unfortunate occurrence and as a result the Zoo has initiated a review to determine the events that led to the deaths," the release said.

The carcasses have been sent to the University of Guelph for a post-mortem.

The Toronto Zoo has 20 wood bison in the conservation program and has been involved in the program for more than 40 years.

The wood bison population in Canada is estimated to be around 10,000 and they are currently classified as threatened.