Updated

Investigators and the parents of a 17-year-old killed by a Siberian tiger at San Francisco Zoo want to know how the cat could have got out of its enclosure — and whether it had help.

Marilza Sousa and her husband, Carlos, choked back tears Wednesday night as they described their shock over the death of Carlos Sousa, Jr. "I wish I was sleeping and this was just a bad dream, but it's not," Marilza Sousa said.

They said they learned of their son's death from the coroner's office, and neither police nor zoo officials had contacted them.

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"They didn't call, like we lost a dog or a cat," Marilza Sousa said. "But we do have questions. How did this happen?"

The zoo was to remain closed Thursday as police continue probing how the animal was able to leave an enclosure surrounded by an 18-foot wall and a 20-foot moat. Police shot the 300-pound animal to death after it killed Sousa and severely mauled two brothers who also were visiting the zoo.

Police Chief Heather Fong said the department opened a criminal investigation to "determine if there was human involvement in the tiger getting out or if the tiger was able to get out on its own."

One zoo official insisted the tiger did not get out through an open door and must have climbed or leaped out. But Jack Hanna, former director of the Columbus Zoo, said such a leap would be an unbelievable feat and "virtually impossible."

"There's something going on here. It just doesn't feel right to me," he said. "It just doesn't add up to me."

Instead, he speculated that visitors could have been fooling around and might have taunted the animal and perhaps even helped it get out by, for instance, putting a board in the moat.

Ron Magill, a spokesman at the Miami Metro Zoo, said it was unlikely a zoo tiger could make such a leap, even with a running start.

"Captive tigers aren't nearly in the kind of shape that wild tigers have to be in to survive," he said. He said taunting can definitely make an animal more aggressive, but "whether it makes it more likely to get out of an exhibit is purely speculative."

The police chief would not comment on whether the animal was taunted.

The same tiger, a 4-year-old female named Tatiana, ripped the flesh off a zookeeper's arm just before Christmas a year ago while the woman was feeding the animal through the bars. A state investigation faulted the zoo, which installed better equipment at the Lion House, where the big cats are kept.

Zoo director Manuel Mollinedo said Wednesday he gave no thought to destroying Tatiana after the 2006 incident, because "the tiger was acting as a normal tiger does." As for whether Tatiana showed any warning signs before Tuesday's attack, Mollinedo said: "She seemed to be very well-adjusted into that exhibit."

It was unclear how long the tiger had been loose before it was killed. The three visitors were attacked around closing time Tuesday on the 125-acre zoo grounds. Four officers hunted down and shot the animal after police got a 911 call from a zoo employee.

The two injured men — brothers aged 19 and 23 from San Jose — were in stable condition Wednesday at San Francisco General Hospital. They suffered deep bites and claw wounds on their heads, necks, arms and hands, said Dr. Rochelle Dicker, a surgeon. She said they were expected to recover fully.

Sousa's parents said they didn't know why their son went to the zoo Tuesday, but it should have been a fun Christmas Day activity.

"It's not a safe place for kids," Marilza Sousa said. "People go there to have a good time, not to get killed."