"I don't think my son would do something like taunt animals," Carlos Sousa told ABC's "Good Morning America." "It's unbelievable, but only the evidence can prove that. And right now I can't say much."
His son, Carlos Sousa Jr., 17, was one of three men attacked by a Siberian tiger around closing time on Christmas. Police shot the 300-pound animal to death after it killed Sousa and severely mauled two brothers who also were visiting the zoo.
According to the San Francisco Chronicle, police found a shoe and blood in an area between the gate and the edge of the animal's 25- to 30-foot-wide moat, prompting the possibility that one of the victims dangled a leg or other body part over the edge of the moat. Police on Thursday could not confirm the Chronicle's report to The Associated Press.
"I don't think this deserves to happen to anybody -- taunting or not taunting," Carlos Sousa told ABC. "Animals should be protected from the people and the people should be protected from the animals."
Police Chief Heather Fong said Wednesday 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."
The zoo was to remain closed Thursday.
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."
Instead, he speculated that visitors could have been fooling around and might have taunted the animal and perhaps even helped it get out by, say, 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 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 zoo has a response team that can shoot animals. But zoo officials and police described the initial moments after the escape as chaotic.
The first attack happened right outside the tiger's enclosure -- Sousa died at the scene. Another was about 300 yards away, in front of the zoo cafe. The police chief said the animal was mauling one of the survivors, and when officers yelled at it to stop, it turned toward them and they opened fire.
Only then did they see the third victim, police said.
The two injured men, 19- and 23-year-old brothers 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 told the AP 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," said his mother, Marilza Sousa. "People go there to have a good time, not to get killed."
Associated Press writers Jordan Robertson, Louise Chu and Terence Chea in San Francisco and Carla K. Johnson in Chicago contributed to this report.