Forty-one stingrays at the Calgary Zoo mysteriously died within one week in May. (CBC)

The mass death of stingrays in an expensive exhibit in May is likely to remain a mystery, the Calgary Zoo says.

The zoo had hoped that extensive toxicology tests would provide an explanation as to why all but two of the 43 animals died, but the zoo said the samples yielded no clues.

Zoo conservation director Cathy Gaviller admitted Wednesday that it's frustrating that there will never be a definite answer as to what killed the cow-nosed rays.

An investigation has ruled out disease, bacteria or water quality as potential causes.

Gaviller says the possibility that someone knowingly poisoned the tank will lead to greater security measures in the future.

The zoo is trying to decide what to do with its almost-new, $250,000 exhibit and whether to restock the tank with rays or another species.