Medical staff and officials at the Calgary Zoo are trying to find out what killed 34 cownose stingrays in the span of just 24 hours.

Just nine stingrays remain of what used to be a school of 43. The animals were part of the Calgary Zoo's newest exhibit, which cost $250,000 to set up. But on Sunday afternoon staff noticed the stingrays were acting strangely and within hours most were dead.

"To say that people are saddened and shocked is an understatement," Cathy Gaviller of the Calgary Zoo told CTV News Calgary.

Zoo officials told CTV Calgary that it took just three hours for 26 of the stingrays to die Sunday. Eight more had died by Monday morning. The cause of their deaths remains a mystery, but there are concerns that the water in the main tank may be to blame.

"Just the pattern of mortality would lead us to believe it's something to do with the water," Sandie Black, the Calgary Zoo's head veterinarian, told The Canadian Press.

"What we're seeing is very, very acute. It's something that's happened over the last 24 hours, so we've been monitoring the health of these animals, watching their food, watching their behaviour."

The stingray exhibit opened in February, drawing thousands of visitors every day. It's now closed while zoo officials try to figure out what went wrong. Meanwhile, the staff is keeping a close eye on the few survivors.

"We're cautiously optimistic that they'll recover," said Dr. Doug Whiteside, the zoo's veterinarian.

The stingray deaths are the latest tragedy to hit the zoo. Three gorillas died last year at the facility. And last fall, a six-year-old hippo died at the zoo, just hours after it had been transported there from a zoo Denver.

Black said the tragedies have "absolutely no relation to each other."

With a report from CTV Calgary and files from The Canadian Press