CHICAGO -- Lincoln Park Zoo officials Thursday afternoon were trying to figure out how an experienced keeper was seriously injured by African lions in an outdoor exhibit designed to prevent direct contact between humans and large carnivores.

The zookeeper, Nancy DeFiesta, 60, called for help on her radio at about 9 a.m., and an emergency response team arrived to find the woman lying at the bottom of a moat in the outdoor lion exhibit, said Dr. Kathryn Gamble, chief veterinarian at the zoo.

Three adult African lions, one male and two females, were standing near DeFiesta at that time, Gamble said at a news conference held in the Judy Keller Education Center at the zoo.

A rescue ladder was dropped down into the moat, and DeFiesta was able to climb out on her own, despite having suffered several bite wounds and lacerations, Gamble said. There is about a 12-foot drop from the rocky exhibit to the bottom of the moat.

Crews immediately worked to contain the animals by discharging a fire extinguisher, the sound of which gives signals to the lions, she added.

Zoo officials were able to speak with DeFiesta, who remained coherent throughout the rescue, and she was in serious condition at Advocate Illinois Masonic, Gamble said.

Though the injured keeper had been speaking from the hospital with zoo officials, she was unable to describe how the accident occurred because she was still in shock, according to Gamble.

"At this point there has been no discussion with her. We're more concerned about treating Nancy and making sure that she's doing well," Gamble said.

At about 11:45 a.m., the three lions were resting calmly inside the Lion House, which is connected to the outdoor area by a series of shift doors and mesh barriers. The exhibit remained open to the public, but visitors were evacuated from the zoo for around a half hour while emergency crews were at the scene.

Gamble said that the barrier devices are in place to prevent direct contact between zoo staff and carnivorous animals. She added that zoo officials were unclear on how the incident happened and would launch an investigation.

"We don't know how the contact occurred at this point," Gamble said. "The exhibit is designed to prevent any direct contact between our keepers and our carnivores."

DeFiesta had been going about her morning routine when the accident occurred, Gamble said. Though she could not provide specifics of the woman's activities, she said her job includes feeding, caring for and training the animals.

Megan Wilson, curator of mammals at the zoo, called the accident "a very, very unusual circumstance."

"We're more worried about (DeFiesta) and her stability with her injuries before we proceed to any investigation directly from her," Gamble said. "We're certainly looking at the exhibit, and the enclosure, and the doors right now."

here is no need for the general public to be concerned for their safety when visiting the Lion House, she added.

Under normal circumstances, heavy-gauge mesh barriers and multiple shifts doors in the exhibit are in place to prevent contact between keepers and the animals, Gamble said, adding that Wilson and her staff were investigating those devices.

"We have a lot of safety procedures in place, including tagging doors that say 'lions in this area' and 'locked,'" Gamble said.

There are also double locks on doors, where keepers leave their keys in locks to indicate where they are, and "double checks" are used between keepers so that they know where each person is within the exhibit, she said.

"That was one of the excellent things about this response. Our keepers knew who was on that run and they knew who else had been in the building," Gamble said.

Wilson and Gamble did not know which or how many of the three lions injured DeFiesta.

Asked if there would be changes in the lions' behavior following an attack, Wilson said she was unable to answer because "this is not something that we have experience with."

"I did see the lions ... we might see some different behaviors, but when I did see them they seemed to be settling down," Wilson said. The lions were not injured.

Gamble and Wilson said they would not speculate on what caused the lions to attack DeFiesta.

The two females each weigh about 240 to 250 pounds, and the male lion weighs about 425 pounds, they said. The pride has been together at Lincoln Park Zoo for about 4 years.

When asked whether the lions could have possibly jumped to a grassy portion of the outdoor exhibit, only inches away from the public, as one zoo visitor told a reporter, Gamble and McGrath said that would be "impossible." If the lion were on that part of the exhibit, it would have jumped out of the exhibit and gotten loose, they added.

They said that lions could not make the jump from the moat to that grassy area, about 15 to 20 feet above. The moat was around 15 to 20 feet wide and stretched the length of the outdoor exhibit. There were also doors at the bottom of the moat.

Zoo spokeswoman Kelly McGrath added that the zoo has not had a serious accident in about 12 to 15 years. She could not provide details, but said the last incident involved an elephant.