The director of the San Francisco Zoo and the city's police chief said Wednesday that they still didn't know how a tiger managed to escape its enclosure on Christmas Day, mauling a teenager to death and seriously injuring two men.

At a news conference just outside the zoo property, Director Manuel Mollinedo said he would bring in outside experts to assess the safety of the outdoor enclosures for lions and tigers.

But Ron Tilson, a tiger expert who is the director of conservation at the Minnesota Zoo and author of a tiger husbandry manual, said the animal probably escaped through an open door.

"For over 200 years tigers have been kept in captivity in European zoos and for over 100 years in American zoos," Tilson said. "No tiger has ever escaped from its enclosure . . . except by human error."

Police Chief Heather Fong said the area had been declared a crime scene while the tiger's means of escape was being investigated. The coroner's office identified the dead teenager as Carlos Sousa Jr., 17, of San Jose.

Police killed the Siberian tiger, Tatiana, shortly after they arrived at the zoo Tuesday.

Asked why the animal was not put down after she attacked a zookeeper last year in the tigers' adjacent feeding area, Mollinedo said: "There was never consideration of putting Tatiana down. The tiger was acting like a normal tiger does."

Tigers are regarded as among the most fearsome, instinctual hunters in the animal world.

"Tigers are alpha predators. They kill and eat animals for food," said Tilson, "including humans."

In the December 2006 incident, Tatiana mauled a zookeeper, Lori Komejan, then 47, in front of horrified visitors.

When Komejan reached down to pull a piece of meat from a drain area just outside the feeding cage, the tiger grabbed the zookeeper's right hand and placed it in her mouth. When Komejan used her left arm to try to push the tiger away, Tatiana grabbed that arm with her claws. Another keeper, attracted by the victim's screams, used a long-handled squeegee to get the tiger to let go.

The California Division of Occupational Safety and Health cited the zoo for the 2006 incident, imposing a fine of $18,000 for what was deemed a "serious" violation of health and safety codes. The zoo has appealed. In its report, the agency criticized the zoo for lapses in protection for animal keepers.

"It was obvious that any of the cats could reach through or under the bars," the report said. Zoo employees "were not given or trained in a procedure that would ensure employee safety and compliance," such as a buddy system or the use of equipment to push a tiger away in case of emergency.

Komejan, who has had several surgeries, filed a claim against the city of San Francisco for $8 million in damages. The legal action is pending, according to the city attorney's office.

Extensive modifications were later made to the tiger feeding area, Mollinedo said.

Sousa, the teenager killed, was a high school junior and a friend of the two injured people, whom the San Francisco Police Department identified as brothers. The station aired an interview with Sousa's father.

"Unfortunately, he was at the wrong place at the wrong time," Carlos Sousa Sr., tearfully told a KPIX reporter. "I miss him very much. I guess the lion or tiger got him in the neck and it killed him instantly. That's what the coroner said."

Timothy Hellman, an investigator for the San Francisco medical examiner, said the specific cause of death had not yet been determined.

The zoo remained closed Wednesday, its gates chained. The only sounds reaching the periphery were trees rustling in the cool breeze and faint yowls from the monkey exhibit. A lone protester, Leigh Lawson, a 25-year-old Humboldt State student, stood watch near the service entrance. Her hand-lettered sign read, "Honk If You Miss Our Tiger."