SAN FRANCISCO — Lions and tigers are being kept hidden from view as the San Francisco Zoo reopens its doors.

The zoo is welcoming visitors for the first time since a Christmas Day rampage by an escaped tiger killed a 17-year-old boy and injured his two friends, brothers aged 19 and 23. Many of those who entered the gates today said they were showing their support for the zoo.

At a bronze tiger sculpture near the zoo's entrance, some laid flowers, cards and photographs of the escaped tiger, who was killed by officers.

Zoo officials have stepped up security following the maulings. They have a new public alert system in place and extra signs warning people not to tease the animals. The big cat enclosure will remain closed indefinitely.

Police are investigating whether the victims had taunted the tiger before it climbed or leaped out of its outdoor pen. The lawyer for the surviving victims says the idea that the men provoked the attack is "nonsense.''