DENVER -- The Denver Zoo was on lockdown just before noon on Saturday after a jaguar attacked and killed a zookeeper who was inside the animal's exhibit.

The zookeeper was taken to Denver Health hospital, where she died of her injuries. Her name has not been released. She had worked at the Denver Zoo for about a year but was an expert with felines.

"She is an experienced zookeeper. She was experienced working with cats. She was a regular keeper in this area," said Denver Zoo spokeswoman Ana Bowie.

The 6-year-old jaguar involved in the attack is named Jorge. He came from the Santa Cruz Zoo in Bolivia in March 2005. The 140-pound cat has never had any problems, officials said.

Denver Zoo officials said a visitor saw the attack at about 11:30 a.m. and told a security worker. An emergency team from the zoo responded with weapons drawn. When they tried to get the zookeeper away from the big cat and tend to her injuries, Jorge came after them, the zoo said.

That's when a zoo employee shot and killed the animal.

Officials said they still do not understand why the zookeeper was inside the exhibit with Jorge.

"The keeper should never have been inside the exhibit with the animal. That's what's particularly unusual here, is how did she end up in that same space with that animal," said Bowie.

There were about 300 visitors inside the zoo when the incident happened. Zoo officials said no one else was ever in any danger.

The zoo was closed for the remainder of the day. It was not known if the zoo would be open on Sunday.

"Obviously, everyone is very upset," said Amy Sarno, with Denver Zoo.

Denver police are investigating the incident.

According to the zoo's Web site, the facility just added a 16-month-old female jaguar named Caipora in December. The female was to be paired with Jorge when she was old enough.

The Web site notes, "The jaguar is the third largest cat in the world behind the tiger and lion. Yet, they have the most powerful jaw of all the big cats."