Officials on Sunday were trying to determine why a zookeeper killed by a jaguar opened the door to the animal's enclosure when zoo policies ban staff members from entering exhibits when big cats are inside.
The Denver Zoo's feline exhibits were closed Sunday for the investigation. Zoo officials also were interviewing staff members to determine what happened.
The zookeeper, 27-year-old Ashlee Pfaff, had opened a door leading from a service area into Jorge's enclosure on Saturday.
A visitor saw the attack from outside the glass enclosure, and his shouts alerted other keepers, zoo spokeswoman Ana Bowie said.
Under zoo policy, staff cannot be in any large cat exhibit when the animal is there.
The 140-pound male jaguar had no history of unusual behavior, Bowie said. Jorge was shot to death by a zoo employee when the animal approached emergency workers trying to save the zookeeper.
One big-cat expert said jaguars are naturally mean and erratic.
"They actually are the most unpredictable animal around," said Nick Sculac, director of Big Cats of Serenity Springs, a rescue center 50 miles southwest of Denver.
The zookeeper died at a hospital about 90 minutes after the attack, zoo officials said. She was bitten in the neck, which was broken, and also suffered severe internal injuries, said pathologist Amy Martin, who performed the autopsy.
Pfaff, who had worked at the zoo for about a year, had undergone regular safety training for the exhibit, shadowed veteran keepers and attended mandatory safety meetings, officials said.
"She was an experienced animal keeper," Bowie said. "This wasn't like it was her first job working with cats."
The jaguar was about 6 years old and had come to the zoo in March 2005 from the Santa Cruz Zoo in Bolivia, the zoo said. Bowie said a necropsy was planned at the zoo's veterinary hospital, but she did not know when.
The zoo added a 16-month-old female jaguar named Caipora in December, and she was to be paired with Jorge when she was old enough, according to the zoo Web site.
A small number of zoo employees undergo frequent training in the use of firearms for such emergencies. Bowie said the zoo has several kinds of firearms and she did not know what type was used to kill the jaguar.
Clayton Freiheit, zoo president and chief executive, issued a written statement saying zoo officials were "deeply saddened" by the incident.
"This keeper was a part of our family and we too are grieving the loss of one of our own."