SAN FRANCISCO, California (CNN) -- The parents of a 17-year-old boy killed by a tiger at the San Francisco Zoo said the attack has forever ruined Christmas for them, while police are investigating whether someone helped the tiger escape.
The victim's parents, Marilza and Carlos Sousa, express shock at the death of their son.
The area of the zoo in which the Siberian tiger killed Carlos Sousa of San Jose has been deemed a crime scene, San Francisco Police Chief Heather Fong said.
The zoo's director, Manuel Mollinedo, said officials have not determined how Tatiana, who weighed more than 300 pounds, escaped from her exhibit area and attacked three patrons Tuesday before police shot and killed her.
Sousa's parents, Carlos and Marilza Sousa, said they were in shock and were having trouble believing what had happened.
"It's hard to believe," said the elder Carlos Sousa, who said he and his wife learned about their son's death Wednesday morning. "I had to go identify the body. It's pretty mangled up." Watch the family's reaction to the attack »
Marilza Sousa put a photograph of her son on the family Christmas tree and said she'd never be able to celebrate the holiday again.
"Our Christmas is with him," she said. "No more Christmas."
Police said Sousa was killed just outside the tiger's enclosure. The two others, who were injured, were about 300 yards away by a cafe.
The two survivors were in stable condition Wednesday and doing well, San Francisco General Hospital said. Watch as the tiger's victims are rushed to the hospital »
The head of the Association of Zoos and Aquariums said this was the first-ever visitor fatality due to an animal escape at a zoo accredited by the association.
Ron Magill of Florida's Miami Metrozoo told CNN that the Siberian tiger is "the most powerful cat on the face of this planet."
Speaking to reporters Wednesday, Mollinedo was asked about an incident last year, in which Tatiana chewed flesh off a keeper's arm during a public feeding demonstration. Mollinedo said that Tatiana "was acting like a normal tiger" at the time, and that the zoo modified procedures to increase safety.
California's Division of Occupation Safety and Health determined the zoo was at fault because of hazardous conditions in the Lion House and lack of specialized safety training for employees. The zoo made changes that the state safety division ordered. The Lion House, the zoo's big-cat exhibit, was closed for more than six months after that incident.
Sousa's sister Beatrice appeared told CNN Wednesday night that the family has not received information about the investigation. "There's a lot of pain. You know, no words for it. It's just too much," she said. "Our family is very, very hurt."
The zoo was closed Wednesday while officials investigated the tiger attack. Mollinedo said the zoo will probably reopen Thursday, but the Lion House will remain closed "until we get a better understanding of what actually happened."
Tatiana was held in an exhibit area that included a 20-foot moat and an 18-foot wall, Mollinedo said. The 4-year-old tiger was born at the Denver Zoo and came to San Francisco in 2005.
Experts from other zoos will inspect the setup to help suggest modifications to assure safety, he said.
"We have deemed the site, as of last night, a crime scene," Fong said Wednesday, and police are working to gather evidence and witness statements.
Fong said fire and police responded to emergency calls shortly after 5 p.m. Tuesday. When police arrived, they saw the tiger "sitting next to a person on the ground," and the tiger turned back and began attacking the person again, she said.
Officers yelled at the tiger to stop, and did not fire immediately "for fear that they would not be able to contain their fire," she said. "When the yelling was occurring, the animal turned toward the officers," who then shot and killed the animal, Fong said. Watch as animal expert Jack Hanna discusses the attack »
Authorities carried out multiple searches to ensure there were no other victims, she said.
The zoo had closed at 5 p.m., and only around 20 people were still there, Mollinedo said.
The other victims of the tiger attack have not been publicly identified.
Dr. Rochelle Dicker, a surgeon at San Francisco General Hospital, said the two "young men" injured are "in very stable condition." In fact, she said, "they look absolutely fantastic."
Doctors are focusing on ensuring that the patients don't develop infections, she said, adding that they "will be on antibiotics for some time.
"By the time they got here, they were very, very stable -- not close to death," Dicker told reporters. "Really, it was just a matter of washing out their wounds."
Jim Maddy, president of the Association of Zoos and Aquariums, issued a statement expressing "sympathy to the family of the guest that lost his life, and our deepest hope that the two injured guests will recover." Read about other escapes and attacks by captive animals
Maddy called the San Francisco Zoo "great," and noted that it is accredited by the association. "Until this incident, there had not been a visitor fatality resulting from an animal escape at an AZA-accredited zoo.
"AZA mandatory accreditation standards require safety and emergency protocols that go beyond federal, state or local requirements. Regular safety training and annual emergency drills are required by these mandatory accreditation standards."
He said association rules "require that the San Francisco Zoo provide a thorough report to its independent accreditation commission, which will review it and determine any actions that need to be taken. We will not speculate on what action might be taken until the facts are fully reviewed."