Investigators at the San Francisco Zoo are trying to determine whether a tiger could have gotten out of its enclosure on its own yesterday, or whether it had some help.
The 300-pound tiger mauled three people, one fatally.
As they search for answers, police are considering the zoo a crime scene. Chief Heather Fong says that's because they're not certain whether the incident involved any "human action."
Police do say they're confident there were no other victims. They searched the zoo grounds during the night, using searchlights and thermal imaging equipment, and again during daylight.
Officials are at a loss to explain how the tiger could have gotten past a 15-foot-wide moat and 20-foot-high walls. One zoo official says it "appears to have climbed or otherwise leaped out of the enclosure."
Animal expert Jack Hanna says if that turns out to be true, other zoos will have to reassess their tiger enclosures.
The tiger was killed by police.