CALGARY -- A Calgary Zoo investigation has pinpointed what caused the death of an otter named Logan last week: a pair of pants that were not meant to be in his enclosure.

The 12-year-old North American river otter was found struggling in a pool last week at the zoo's Canadian Wilds exhibit. A keeper jumped in to save him, but the animal did not survive.

A necropsy revealed Logan drowned after becoming tangled in the clothing.

Keepers give animals "enrichment" items that are meant to elicit behaviours that they would display in the wild, zoo general curator Colleen Baird said Wednesday.

She said the zoo has rules about which items are appropriate for specific animals. In the otter's case, the pants were not authorized.

"Each species is different. An enrichment item that might be OK for one species would not be OK for another," she said.

Two staff members have been disciplined, but Baird cited privacy reasons in declining to say what exact measures are being taken.

"When an animal dies, we take this very seriously and it's very tragic," she said. "The zoo is heartbroken over this."

Enrichment items that would be allowed in the otter's pool include clams and mussels, pipes and tubes for rafts and hoses to create bubbles to mimic a flowing river.

Some types of clothing and fabric are allowed for certain animals -- ferrets and gorillas, for example, said Baird.

The zoo has three remaining otters -- a one-year-old male named Finnegan, a 16-year-old male called Callebaut and a 12-year-old female that staff call Charlotte.

So far, they appear to be doing fine following Logan's death.

"We're monitoring closely to make sure that we can see if there are any changes or effects due to this loss and we're just keeping a close eye on what they're up to."

Baird said the zoo's animal protocols are among the most detailed in Canada. It plans to reinforce its rules with staff to ensure nothing like this happens again.

The zoo has made headlines in the past over the deaths of its animals. They include about 40 stingrays in an interactive exhibit in May 2008, a giant capybara crushed by a hydraulic door in 2009 and a caribou calf from complications related to sudden severe neck trauma in 2014.