SAN FRANCISCO (AP) - The big cat exhibit at the San Francisco Zoo was cordoned off as a crime scene Wednesday as investigators tried to determine whether a Siberian tiger that killed a visitor escaped from its high-walled pen on its own or got help from someone, inadvertent or otherwise. Police shot the 300-pound animal to death after a Christmas Day rampage that began when the tiger escaped from an enclosure surrounded by what zoo officials said are an 18-foot wall and a 20-foot moat. Two brothers who also were visiting the zoo were severely mauled. Police Chief Heather Fong...