The grieving father of the teenager killed by an escaped tiger at San Francisco zoo yesterday rejected speculation that his son may have provoked the attack by dangling a leg over the moat.

"I don't think my son would do something like taunt animals," Carlos Sousa Sr told ABC's "Good Morning America." "It's unbelievable."

Carlos Sousa Jr, 17, was killed by the rare Siberian tigress, named Tatiana, in a Christmas Day rampage that also left two friends, brothers aged 19 and 23, with serious wounds.

Mr Sousa Sr later told the San Jose Mercury News that one of his son's friends might have provoked the animal. "It could have been one of the other kids. And my kid could have just gotten it the worst," he said.

Police have declared San Francisco zoo a crime scene as they investigate a possible human role in the animal's escape from its moated enclosure by the Lion House.

Zoo officials insist it would be impossible for the 300lb big cat to scale the enclosure's 18' high wall or leap the 25' moat on its own.

The three friends from nearby San Jose were visiting the zoo together when the attack took place shortly before closing time.

Officials speculate that one of the victims may have dangled a leg or another body part over the enclosure's 14'-deep moat and that Tatiana seized it and climbed out.

Police sources told the San Francisco Chronicle that a search found "a shoe and blood in an area between the gate and the edge of the ... moat."

Other evidence found at the scene included a footprint on a metal fence, indicating that someone had climbed it to get closer to the big cats.

Pinecones and sticks were also found in the moat, where they would not have normally landed, suggesting they may have been thrown at the animal.

"Somebody created a situation that really agitated her and gave her some sort of a method to break out," Manuel Mollinedo, the zoo's director, told the newspaper. "There is no possible way the cat could have made it out of there in a single leap. I would surmise that there was help.

"A couple of feet dangling over the edge could possibly have done it," he said.

Investigators believe all three young men were present when Tatiana killed Mr Sousa Jr with a slash to the throat just outside the enclosure.

The two other men apparently fled, but the tiger tracked them, possibly following a trail of blood, and cornered them by the zoo's Terrace Cafe about 300 yards away.

Four police officers arrived as the tiger resumed its attack and gunned down the animal as it turns its attention to them.

The injured brothers, who have not been named, remained in hospital yesterday after receiving surgery on their wounds.

Mr Sousa Sr, the dead man's father, said he was too busy mourning to have visited the injured men in hospital to hear their account.

But he said: "I don't think this deserves to happen to anybody taunting or not taunting. Animals need to be protected from the people and people need to be protected from the animals."

Tatiana's rampage was the second time the 4-year-old tiger had attacked a human in just over a year. Three days before Christmas 2006, Tatiana chewed the flesh off a zoo-keeper's arm during a public feeding session.

The zoo has no video cameras monitoring the enclosure outside the Lion House, also home to a male Siberian tiger named Tony and three Sumatran tigers.

Police will try to use physical evidence found at the scene as well the wounds to the victims and an examination of the dead tiger to determine what happened.

Investigators will have to establish why Tatiana apparently ignored other visitors to pursue the two injured victims 300 yards to the cafe.

Meanwhile, vandals freed two cougars from their cages at Lincoln Park Zoo in Wisconsin in a possible copy-cat attack. The two cougars were recaptured without injury.

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