SAN FRANCISCO — San Francisco police are investigating the possibility that one of the victims in the fatal tiger mauling on Christmas Day climbed over a waist-high fence and then dangled a leg or other body part over the edge of a moat that kept the big cat away from the public, sources close to the investigation said Wednesday.

The minimal evidence found at the scene included a shoe and blood in an area between the gate and the edge of the 25- to 30-foot-wide moat, raising questions about what role, if any, the victims might have had in accidentally helping the animal escape.

The three victims, all young men from San Jose, were visiting the zoo together. They were all present just outside the tiger’s grotto when the tiger escaped, killed 17-year-old Carlos Sousa Jr. with a savage slash to the throat, and injured the other two. The names of the survivors, who are brothers ages 19 and 23, have not been released.

The injured victims fled, leaving a trail of blood, which police believe the tiger followed for 300 yards up a zoo pathway. As the tiger cornered and attacked one of the brothers, four police officers arrived, distracted the animal and shot it dead.

What followed were hours of confusion as police attempted to find other tigers that zookeepers believed might have escaped.

When police arrived at the zoo, they were hampered by a lack of emergency lights, video surveillance cameras and maps. They didn’t know how many animals might be on the loose or how many victims might have been injured.

And though they found and killed the escaped tiger within minutes of their arrival, they had been told three other tigers might be on the loose.

Zoo officials said they are trying to figure out how the 4-year-old Siberian tiger named Tatiana escaped from her enclosure shortly after the zoo’s 5 p.m. closing time, while there were still many visitors walking around the zoo. No doors to the grotto had been left ajar, they said. The animal is the same one that attacked a zookeeper almost exactly a year ago.

Police sources said a footprint had been found on a metal fence, suggesting that someone had climbed the fence to get closer to the big cats. Authorities were looking into whether the tiger escaped by latching on to a leg or body part.

Zoo director Manuel Mollinedo said it was also likely that the animal was provoked.

“Somebody created a situation that really agitated her and gave her some sort of a method to break out,” Mollinedo said. “There is no possible way the cat could have made it out of there in a single leap. I would surmise that there was help.

“A couple of feet dangling over the edge could possibly have done it.”

Sources said pinecones and sticks that were found in the moat might have been thrown at the animal. Those items could not have landed in the grotto naturally, they said.

However, police Sgt. Neville Gittens maintained that there was no reason to think that the victims were taunting the tiger.

The two survivors were in serious but stable condition Wednesday following surgery at San Francisco General Hospital, doctors said.

The zoo, which closed Wednesday, will remain shut today so that police could continue their investigation. Mollinedo said he hopes the zoo will reopen Friday, but the tigers and lions will not be on display for at least a week.

The hunt for the escaped tiger Tuesday was chaotic, with police and paramedics frantically scrambling through the 1,000-acre zoo grounds to locate victims after discovering Sousa’s body.

There are no surveillance cameras pointed at the big cats’ grottos, so officers could not find out from zoo guards which animal or animals had escaped. Initial calls for help didn’t say what sort of animal had escaped, sources said. The lack of cameras also has made it difficult to ascertain how the attacks occurred.

Police said the investigation will focus on physical evidence collected at the zoo, witness statements, an autopsy of the victim and the necropsy of the tiger.

New cameras and a metal barrier will be installed to protect the public from the tigers and lions, Mollinedo said.

As darkness deepened Tuesday, officers found the zoo lighting was insufficient to illuminate the trees, foliage and public areas where they feared that more animals might be lurking or more victims might be found. Firefighters standing on tall ladders outside the zoo illuminated the grounds with portable lights to help the search.

“We didn’t know how many tigers were out,” Mollinedo said. “I was under the impression we had two or three cats escaping. There was a sense of paranoia around here.”

Investigators canvassed the zoo for hours Wednesday morning before finally determining that nobody else had been hurt.

A Fire Department source said three paramedics found Sousa’s body Tuesday, with a wide gash across the neck, lying near the grotto. The enclosure is separated from the public by a 25- to 30-foot-wide moat — a trench that is 14 feet deep.

The paramedics and an ambulance driver were told of additional victims outside the Terrace Cafe restaurant, about 300 yards east. When they arrived, they found the tiger standing over one of the brothers.

The paramedics and driver were soon joined by the four police officers, who distracted the tiger with the red lights of their two patrol cars before shooting her to death with their .40-caliber handguns.

Gittens said the officers did not want to shoot the animal while she was sitting next to the victim.

“I can only imagine the patrons walking around, and suddenly seeing this tiger,” he said. “It was probably surreal.”

Police were treating the zoo as a crime scene Wednesday. Yellow police tape surrounded the area near the cafe where the animal was slain.

It’s not known why the tiger singled out the three visitors or why she tracked down the two wounded victims, apparently ignoring other zoo patrons.

Rochelle Dicker, an emergency room surgeon at San Francisco General Hospital who operated on the victims, said Wednesday that the two survivors were recovering remarkably well. They were awake and stable, Dicker said.

Doctors are focusing on preventing infection from the bacteria that might be present in the men’s wounds, she said. Those bacteria are similar to the type found in the common house cat, officials said.

The zoo was eerily quiet on the day after the tragedy. The cafe where the attack victims were found was shuttered and still, and a dozen police officers gathered with zoo officials nearby, poring over maps and handing out equipment. Zookeepers held a staff meeting and also met with grief counselors.

Visitors continued to filter up to the entrance, unaware of what had happened.

“Oh my gosh! I hadn’t heard anything about a tiger,” said one visitor, Komer Poodari of San Jose. “I guess we’ll go to Fisherman’s Wharf.”

Mollinedo said that the zoo has a response team armed with tranquilizers and firearms, but that the scene unfolded “so quickly that the officers found Tatiana first.”

This is the second time in just over a year that this same tiger attacked a human.

On Dec. 22, 2006, Tatiana chewed the flesh off zookeeper Lori Komejan’s arm in front of about 50 visitors lingering in the Lion House after the cats were fed. A state investigation later ruled that the zoo was at fault for the attack because of the way the cages were configured.

“There was never any consideration for putting her down. The tiger was acting like a normal tiger,” Mollinedo said.

The public feeding demonstrations at the Lion House resumed in September after about $250,000 in safety upgrades. The city, which helps fund the zoo, has been sued by Komejan and is assessing whether it is liable for the Christmas Day mauling.

Mollinedo has brought in colleagues from other accredited zoos to do a thorough analysis of the big cat exhibits.

“We want to make sure they are safe and see what kind of modifications should be done to ensure the safety” of people and animals, he said.

The U.S. Department of Agriculture will “in most likelihood” investigate the zoo’s tiger facility, said agency spokesman Jim Brownlee. Inspectors from the USDA’s Animal and Plant Health Inspection Service, which oversees the nation’s zoos, will “probably be on location very soon,” Brownlee said Wednesday night. “We would look at the whole situation in a comprehensive manner,” he said.

Keepers at other U.S. zoos said they were waiting for more details about the San Francisco attack before deciding whether to change their tiger enclosures or security procedures. Moats and sheer walls are common methods of protecting visitors from large animals at outdoor zoo exhibits.

“We still don’t know what happened,” said Tony Vecchio, director of the Oregon Zoo in Portland. “We know what animals can do, and we build barriers that are taller and wider than that. But one of the rules in the zoo business is that animals will always surprise you.”