CHICAGO (AP) — Officials say all 54 stingrays in a popular exhibit at a suburban Chicago zoo died because the oxygen levels in their tank dropped too low.

The Chicago Zoological Society released a statement Saturday saying the four southern stingrays and 50 cow-nose rays in the Brookfield Zoo exhibit were in a shallow pool that allows visitors to touch and feed the creatures. Most died Friday afternoon.

Bill Zeigler, a senior vice president at the society, says zoo staffers worked hard to try to save the rays and are devastated by their deaths.

The society says the staff is working to get the tank’s oxygen levels back to normal and that the seasonal exhibit will remain closed for the rest of the summer.