Police investigating the killing of one man and the mauling of two others by a rare Siberian tiger at San Francisco Zoo suggested last night that the animal may have had human help to escape.

The tigress, named Tatiana, got out of her moated enclosure on Christmas Day. She was shot dead by police.

The attack came almost exactly a year after the 300lb (136kg) beast mauled a keeper’s arm through the bars during a public feeding session.

Police declared the zoo a crime scene yesterday so that they could investigate whether anyone had helped Tatiana to escape.

“The purpose of the criminal investigation is to see if the tiger was able to get out on its own or whether there was human involvement,” San Francisco Police Chief Heather Fong said.

Alerted by frantic emergency calls, police armed with shotguns cordoned off the zoo shortly before closing time on Tuesday and scoured the densely populated area around it with helicopters in case other tigers had escaped from their pen.

Tatiana was discovered by four officers with one of her bloodied, but still alive, victims.

“When the officers first approached this other victim in front of the café, the tiger was sitting right next to the victim. As the officers approached, the tiger continued its attack,” Sergeant Steve Mannina, a police spokesman, said.

“As the officers moved even closer, the tiger focused its attention on the officers and started coming towards the officers. That is when the officers fired.”

Siberian tigers are an endangered species, with fewer than 400 surviving in the remote forests of Russia’s Sikhote-Alin mountain range, east of the Amur River. Another 600 are kept in captivity.

Four-year-old Tatiana was moved to San Francisco Zoo from Denver in the hope that she would mate with a male called Tony.

The tigress lived with Tony and three Sumatran tigers in outdoor enclosures, kept away from the public by a 4½meter (15ft) wide moat.

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