Officials with the Assiniboine Park Zoo said an older Siberian male tiger pushed open a mistakenly unlocked gate and stepped into a neighbouring den housing two younger male cats.
One of the younger tigers then attacked the older one and, although the fight took place at the rear of the enclosure, it happened in view of any visitors who happened to be at the tiger exhibit.
Margaret Redmond, president of the Assiniboine Zoo Conservancy, told reporters that the students were on a class trip with a teacher and saw the beginning of the attack.
"We were able to get them away before they really witnessed anything too unfortunate."
No visitors were in any danger at any time, she said, and a review of the incident is underway.
The zoo's chief veterinarian, Dr. Chris Enright, said the 19-year-old tiger who died was named Baikal and had been at the zoo for about five years.
He lived in an enclosure that's connected to a second enclosure by a gate and corridor. The other den houses the pair of younger tigers, two-year-old brothers named Samkha and Vasili, who were transferred from the Calgary Zoo last year.
Enright said tigers are solitary as adults and that's why Baikal lived by himself. The two brothers had grown up together and were still young enough to be together.
Vasili, the one who killed Baikal, was likely defending his turf, said Enright.
"Any tiger — they're a 350-pound predator — has the potential to act aggressively in defense of itself," he said.
"It is a tiger defending his territory against another male that he would only see as a rival. You can't fault a tiger for being a tiger."
Brian Joseph with the zoo's operations department said staff are devastated by Baikal's death because, like any animal at the zoo, he was like a family member.
He said animals have killed each other at the zoo before, but never because of human error.
Baikal had chronic health problems due to kidney disease and, at his age, was considered elderly.
"Two years ago it wasn't expected it would make it this far," said Joseph. "It's pretty amazing he felt so inspired that he wanted to go have a little match-up with the younger cats."
Wildlife experts say Siberian tigers, also known as Amur tigers, are the world's largest cats. They live mainly in eastern Russia's birch forests, though some exist in China and North Korea.
Once hunted to the brink of extinction, there are an estimated 400 to 500 left living in the wild.
— By Chris Purdy in Edmonton with files from CJOB.