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Whale shows at SeaWorld were canceled Thursday, and officials were re-evaluating safety procedures a day after a 12,000 pound killer whale grabbed a trainer's ponytail, dragged her under water and killed her in front of shocked onlookers at Shamu Stadium.

Dawn Brancheau, 40, was "pulled underwater for an extended period of time," by the whale, Chuck Tompkins, SeaWorld's curator of zoological operations, told CNN's "American Morning." He said he had no further information on the exact cause of Brancheau's death, citing an ongoing investigation.

The incident occurred about 2 p.m. Wednesday. Tompkins said the whale, named Tillikum, had just finished a session with Brancheau, who was standing by the side of his pool and leaning over the whale, rubbing his head.

"She had a long ponytail that brushed in front of her and apparently got in front of his nose," Tompkins said. "He probably felt it." Tillikum grabbed the ponytail and pulled Brancheau into the water, he said.

Earlier accounts varied on how Brancheau ended up in the tank.

A witness told CNN affiliate WKMG-TV that the whale approached the glass side of the 35-foot-deep tank at Shamu Stadium, jumped up and grabbed Brancheau by her waist, shaking her so violently that her shoe came off. A SeaWorld employee, who asked not to be identified, described the incident the same way.

Orange County Sheriff's Office spokesman Jim Solomons said Brancheau slipped into the tank.

Tillikum has been linked to two other deaths. He and two other whales were involved in the drowning of a trainer at a Victoria, British Columbia, marine park in 1991. The trainer fell into the whale tank at the Sea Land Marine Park Victoria and was dragged underwater as park visitors watched.

In 1999, Tillikum was blamed for the death of a 27-year-old man whose body was found floating in a tank at SeaWorld, the apparent victim of a whale's "horseplay," authorities said then.

The Orange County Sheriff's Office said the man apparently hid in the park until after it closed, then climbed into the tank.

The 22-foot-long whale was "not accustomed to people being in his tank" and "wouldn't have realized he was dealing with a very fragile human being," Solomons said at the time.

Because of Tillikum's history, as well as his size, trainers did not get into the water with him, Tompkins told CNN. Specific procedures were in place for working with him, he said, although "obviously, we need to evaluate those protocols."

"He's just a really, really large animal," Tompkins said, noting that female killer whales weigh 6,000 pounds -- half of Tillikum's weight. "Just because of his size alone, it would be dangerous to get in the water with him." But the whale's previous incidents were also taken into account, he said.

Tompkins pointed out that the 1991 incident occurred before SeaWorld owned Tillikum and that no one is sure what took place in the incident eight years later.

Tillikum could have been trying to play with Brancheau or get her attention or companionship, said Nancy Black, a marine biologist who has studied whales for 20 years. Such whales play with seals and sea lions in the wild, tossing them in the air, she said. But they do not kill them and end up letting them go.

"I don't believe the killer whale purposely intended to kill the woman," she said. "It was more likely an accident, I would guess." But, she said, the whale could also have been frustrated for some reason.

Tompkins said there were no indications of any problem with Tillikum or any other animal just before the incident, and that Brancheau "had done a great session with him ... he seemed to enjoy what he was doing at the time."

The incident, however, raises larger questions regarding the captivity of wild animals.

A spokesman for People for the Ethical Treatment of Animals called the death "a tragedy that didn't have to happen."

Jaime Zalac said the organization had called on SeaWorld "to stop confining oceangoing mammals to an area that to them is like the size of a bathtub, and we have also been asking the park to stop forcing the animals to perform silly tricks over and over again. It's not surprising when these huge, smart animals lash out."

Black told CNN that killer whales in the wild live in family groups, and males stay with their mothers their entire lives. Family members rely on each other for social structure and play, and they cover hundreds of miles of ocean, she said.

"I think they do need more space, and situations like that do cause a lot of stress for them, most likely." She said Tillikum had a "flopped fin," something seen in captivity but not much in the wild.

But Tompkins said, "We have a tremendous track record with these animals at SeaWorld" and a very small percentage of problems. It's useful to have animals in the park, he said, because it gives scientists a chance to study them and gives members of the public an opportunity to see them and learn about them.

"This is the first time in 46 years that we've ever had an incident like this with a trainer," he said. Although Tillikum is large and has to be handled carefully, "to mark him as a killer is unfair."

In 2006, a trainer at the adventure park was hospitalized after a killer whale grabbed him and twice held him underwater during a show at Shamu Stadium.