As the San Francisco Chronicle reports:
Zoo officials are still unsure how the tiger escaped the enclosure. Authorities believe it initially attacked all three victims, killing [17-year-old Carlos Sousa Jr.]. Officials believe the cat then followed blood trails to Terrace Cafe, where it cornered the other boys, brothers ages 19 and 23.
Although some zoo officials speculate the threesome may have teased the tiger, San Francisco police Lt. Leroy Lindo said police currently have no reason to believe the three men taunted the animal prior to the attack, which happened shortly after the zoo's 5 p.m. closing time. Dozens of visitors and some employees were still inside the zoo at the time.