CALGARY -- The Calgary Zoo suspects the 39 cownose rays that have mysteriously died in recent days were poisoned.

"This is a huge tank, so something substantial had to have been put in ... we don't know if it was a powder or a liquid," said zoo spokeswoman Laurie Herron. "But at this point we're thinking it was not accidental."

Tests from an independent lab released early Wednesday suggested the water chemistry of the ray pool was within acceptable ranges. Analysis included checking levels of ammonia, dissolved oxygen, PH, salinity and other natural minerals.

Late Sunday, 26 of the 43 rays housed in the pool died, with the other deaths following over the following days. Only four rays remain.

"It's disturbing to think of this as a possibility, but we have so far been unable to pinpoint any other cause that would provoke such an acute and severe reaction in the rays."

The ray pool, which allows visitors to reach in and touch them as they swim, was introduced at the zoo in February.