Staff at Winnipeg's Assiniboine Park Zoo are trying to understand what led to the death of a young polar bear.

"They're taking it really … it's tough for them," said Dr. Chris Enright, the zoo's head of veterinary services, pausing to find the right words.

"Eli was a young animal. Eli was a bear that has been with us for the past year and a half and really, had a second chance at life with our staff and the other bears here and with the public.

"It's hard on all of them when an animal passes away."

The almost-three-year-old cub wasn't acting like itself on Friday, and didn't have much of an appetite, Enright said. Eli was significantly worse on Saturday so a decision was made to put him under anesthetic to enable the vets to conduct a closer examination and collect some samples.

"Unfortunately, he experienced difficulty breathing and passed away," Enright said, adding the samples will be sent away for further diagnostic testing.

Poring over video footage

The investigation into the death is still in a preliminary stage, but doctors found significant swelling around his pharynx, at the back of his throat, and continuing down his neck along the trachea.

What caused that, however, is still unknown.

"At this point, we have a list of possibilities and further testing should help us whittle away at that list to get to one final answer," Enright said.

"We're looking at the [video] footage, because we have numerous cameras on our polar bear exhibits. We're looking at an impact or trauma as a possibilty."

However, there are no indications of any external punctures to suggest an attack from another bear, and no internal punctures that could have been made by jagged bones ingested by Eli, Enright said.

When news of Eli'​s death came out on Sunday, Zoocheck Canada, a national animal protection charity, expressed concern there could be something toxic in the enclosure.

Enright said that doesn't appear to be the issue.

"It's absolutely something that we look at closely whenever we have an unexpected incident of illness or mortality," he said. "We don't, at this point, think that there's anything that could spread to the other bears or other animals in the zoo."

Eli arrived at the zoo's polar bear conservation centre as a one-year-old cub with his brother, York, in October 2015.

They were orphaned when their mother was accidentally hit by a cracker shell someone used to scare her and the cubs away from a building entrance in Churchill.

Manitoba Conservation determined the cubs were too young to survive on their own and had them transferred to the zoo's Leatherdale International Polar Bear Conservation Centre. After a quarantine period, they were eventually added to the zoo's 10-acre Journey to Churchill enclosure with the other polar bears.

"We have a great facility and great staff, and we really put that effort into looking after our bears the best way we know how. And I do think our bears here have a good quality of life," Enright said, knowing there will be criticisms from people opposed to zoos.

According to an assessment by Manitoba Conservation, Eli and York would likely have died if they remained in the wild, he said.

There are now eight polar bears living in the Journey to Churchill exhibit.