Ananova:

A US zoo remained closed to visitors as investigators tried to determine how a tiger escaped from its enclosure to attack three visitors, killing one of the men and mauling two others.

Police said they did not expect to find any other victims, but wanted to conduct a thorough sweep of the grounds because it was unclear how long the tiger had been loose on Christmas Day before she was killed by officers.

"There's no better light than daylight," said police spokesman Neville Gittens at San Francisco Zoo

"The idea was to come back and quadruple check to make sure nobody out's there. We just want to know."

The tiger, a female named Tatiana, was the same animal that ripped the flesh off a zookeeper's arm just before Christmas 2006. An investigation of that incident by the California Division of Occupational Safety and Health blamed the zoo, which beefed up the pen where big cats are kept.

The three men - one of them 19 years old and the others in their early 20s - were attacked yesterday on the east end of the 125-acre zoo grounds near Ocean Beach, another police spokesman, Steve Mannina, said.

They suffered "pretty aggressive bite marks," Mr Mannina said.

The officers who hunted down and shot the 300-pound animal were alerted through an emergency rescue call placed by a zoo employee.

The two injured men, ages 19 and 23, were upgraded to stable condition at San Francisco General Hospital after undergoing surgery to have their wounds cleaned and closed, said surgeon Rochelle Dicker. They suffered deep bites and claw cuts on their heads, necks, arms and hands.

Dicker said they were shaken up emotionally and would remain hospitalised for the day, but that because of their youth they would make a full recovery.