A holiday trip to the zoo became an excursion into terror on Dec. 25.
Three men who went to see captured wildlife, suddenly became the prey of a raging tiger.
How the animal escaped is a mystery.
We do know that one victim is dead, with two others lucky to be alive.
The Christmas Day mauling came just as the zoo was closing.
One zoo visitor was already dead, and two others seriously mauled when police and emergency workers arrived and confronted the 350-pound Siberian tiger, "Tatiana."
"As that tiger was attacking someone they shot the tiger to save that person's life," said Lt. Ken Smith, of the SFFD.
A doctor who treated the injured victims said they're in serious but stable condition this Dec. 26, and should survive the ordeal.
"The wounds were primarily on the head and neck, and then on the upper extremities of the two patients, the arms and the hands," stated Dr. John F. Brown, from the San Francisco General Hospital.
Almost one year ago, Tatiana reached a paw through her cage bars and badly injured a zookeeper during a routine public feeding.
Changes in the cages were ordered, and made, long before the Dec. 25 attack.
"There was no way out through the door. The animal appears to have climbed or otherwise leapt out of the exhibit," explained Robert Jenkins, the San Francisco Zoo Director of Animal Care and Conservation.
That means Tatiana - a 4-year old tiger, not fully matured - would, somehow, have to have climbed over 20-foot walls, and across a 15-foot moat to escape.
On Good Morning America, noted zoologist Jack Hannah was asked if a young tiger could accomplish such a feat?
"It's almost like an athlete. We see them break records every year. That would be an incredible feat, but could it be done? I'm not going to say it couldn't be done," added Hannah.
Exactly how it was done will be the question San Francisco zookeepers will have to answer.
The zoo is closed Dec. 26 while officials investigate--and search the zoo grounds, to make sure Tatiana took no other victims.