One of the Assiniboine Park Zoo's Amur tigers died in its enclosure Thursday morning after a fight with another tiger, and officials said some high school students may have witnessed the incident.

"This is a heartbreaking loss for our staff and our visitors," said Brian Joseph, director of zoological operations for the Assiniboine Park Zoo.

Baikal tiger Winnipeg

Baikal (pictured) was fatally injured Thursday morning after getting into a fight with the younger Amur tiger, Vasili.

Zoo officials said a gate was mistakenly left unlocked. Baikal, a 19-year-old adult male tiger, found its way into a new enclosure, which was housing the zoo’s two younger male tigers named Samkha and Vasili.

Baikal and Vasili got into a fight that ended in Baikal's death earlier this morning.

Scott Kwasnitza, Lord Selkirk School Division superintendent, told CBC a group of about 25 high school students around 16 years of age from Lord Selkirk Regional Comprehensive Secondary School were near the enclosure at the time.

Kwasnitza said he thought most of the students saw the fight.

He said because it's difficult to know how students would react to what they saw, they were later offered services from the school's guidance and counselling department as a precaution.

Samkha and Vasili tigers Winnipeg zoo

Baikal made his way into a new enclosure where brothers Samkha (left) and Vasili (right) were being held after a gate to a tunnel connecting the two areas was left unlocked. (Assiniboine Park Zoo)

Kwasnitza sent a letter home to parents.

"Obviously we wanted them to be aware of what the incident was and we did, we talked about the incident," said Kwasnitza. "We said that the students had witnessed a fight between the two tigers, and that the safety of the students was never compromised.”

Another zoo visitor named Robert told CBC he and his young son and wife were at Baikal's enclosure just before the fight.

They went to get lunch at the Tundra Grill in the zoo around 11 a.m. when they heard a noise.

"We heard some crazy rattling of the cages," he said. "It was a bit of commotion that we heard, but we don't know exactly what it was."

When the family tried to return 40 minutes later, zoo staff wouldn't let them near the area.

'Just a tiger being a tiger,' says zoo vet

Doctor Chris Enright, a veterinarian at the zoo, said Vasili is not to blame.

"For the tiger, it was just a tiger being a tiger," said Enright. "It was a tiger defending his territory against another male that he would only see as a rival. You can't fault him for being a tiger."

Baikal had lived at the Assiniboine Park Zoo for the last five years.

With the loss of Baikal, three Amur tigers remain at the zoo: Kendra, an adult female, as well as two-year-old brothers Vasili and Samkha.

The brothers were transferred to the Winnipeg zoo from Calgary in January.

Zoo officials are investigating the confrontation to ensure it doesn't happen again.

"Our zoo keepers are well practised and well trained in checking and double checking locks," said Enright. "At this point we don't know how this particular incident happened."

This is the second tiger death at the zoo in two years.