Zoo officials say a toxin in the tank may be responsible for killing 40 stingrays. The remaining three rays are being closely monitored. (CBC)

Five more stingrays have died, the Calgary Zoo announced Wednesday as it asked for police assistance with its investigation into the mysterious and sudden deaths of all but three of its rays.

The zoo said the deaths were likely caused by a poison in the tank.

In a release, the zoo said it has contacted the Calgary Police Service for help "in moving forward with its efforts to determine the cause of the rays' death."

Calgary police Staff Sgt. Greg Johnson told CBC News on Wednesday that the police department is only responding to a request from the zoo, adding he has no evidence or suspicions that any criminal activity has taken place.

"These are amazing creatures and it's unfortunate that this incident occurred, so we give it whatever priority that we feel is necessary," Johnson said. "That's why we were called in today."

But he added that police action so far is limited to opening a file on the case and assigning it to an investigator.

Test results released Wednesday showed the water chemistry of the pool "was within acceptable ranges," the zoo said in a release.

The tests are not able to identify possible toxins in the water, and further tests are being conducted.

"The likelihood certainly is that something got into the water that was toxic to the fish — that's our most likely theory at the moment," Cathy Gaviller, the zoo's director of conservation, research and education, said before the latest results were released.

The exhibit opened in mid-February, after the rays were shipped in from Florida, and in April visitors were able to touch and feed the creatures.

On Sunday, after a public visit, the ray keeper noticed some of the cownose rays were swimming erratically and seemed distressed.

By Monday, 34 rays had died. Another one was announced dead on Tuesday, four more died overnight and one on Wednesday.

The remaining three rays were being held in an oxygenated tank under constant medical supervision.

Poisoning a possibility: professor

Galliver said a toxin added to the water would be able to trigger such sudden deaths.

"Rays are fish, and like any other fish, I mean, they're closely associated with their aquatic environment. I mean, much more so than we are with the air we breathe," she explained.

"So it takes a very, very small amount of many available things, you know, pesticides, herbicides, chemicals, household chemicals, any number of things could be quite toxic to the fish."

Chris Lowe, a professor of marine biology in California, said rays are a hearty animal and the sudden deaths signal poisoning is a likely possibility.

"Because the animals all responded at about the same time, and showed similar symptoms, it suggests that it's probably some toxicant, potentially a poison or something like that, was introduced to the tank," he said.

The zoo said the exhibit will remain closed until further notice. The neighbouring elephant exhibit will also be closed.