Just nine cownose stingrays survived whatever happened Sunday afternoon and the exhibit remained closed to the public Tuesday, the Globe and Mail reported.

Zoo veterinarian Doug Whiteside performed necropsies on the young rays, which weigh about 2 pounds and measure 12 inches across. He said their breathing gills appeared irritated, meaning something toxic could have gotten into the 10,000-gallon tank.

"It's hard to speculate but certainly you have to look at was something introduced through the water lines? Was something introduced into the pool?" Whiteside said to the Globe and Mail. "I don't think there is anything innocent that would be able to kill that many rays in a tank of that size."

The zoo's water tests showed all normal chemical and oxygen content and proper temperature but toxicology samples sent to an outside lab could take several weeks to be analyzed, the report said.