The bear passed by without harming the zookeeper, who was cleaning up toys before the June 24 incident and thought the 1-year-old polar bear named Payton was safely locked away.
Brookfield spokeswoman Sondra Katzen said zoo patrons were never in danger, and the zookeeper was rescued from the exhibit within minutes.
Katzen said the polar bear should have been locked in the den when the zookeeper went into the exhibit, but the animal was not.
"So, obviously, the lock wasn't checked, " she said.
Katzen said checking the locks should have been part of the zookeeper's daily routine.
The incident occurred less than two weeks before a gorilla at Lincoln Park Zoo attacked and bit an intern, sending the woman to the hospital with minor injuries. That incident, which occurred Tuesday, is under investigation by both the Lincoln Park Zoo and the U.S. Occupational Safety and Health Administration.
Katzen said OSHA was not notified in the June 24 incident at Brookfield because no one was injured.
She said members of Brookfield Zoo's life safety and animal programs departments are reviewing what happened and have interviewed the keeper involved. It has not been determined if any disciplinary action will be taken against the woman, Katzen said.
The incident occurred just before noon on a Friday, when the keeper went into Payton's exhibit to remove some toys, Katzen said.
The keeper, who has experience in caring for the zoo's polar bears, could see Payton in his den area and thought the door between the den and the exhibit area was locked, Katzen said. She said the keeper removed some of Payton's toys, then went back to remove some more.
At about that time, Katzen said, the polar bear apparently pushed through the unlocked door and passed by the keeper at a distance of about 10 feet. The polar bear then went into a separate alcove area, while zoo officials scrambled to help the keeper.
Katzen said an emergency response team arrived within one minute and quickly got the keeper out of the exhibit by putting a ladder across the moat.
"We do go through drills throughout the year, " Katzen said. "There is a protocol and a plan in place that is followed when incidents happen. Staff responded very professionally and according to what they were supposed to do."
Katzen said that in the 18 years she has been at Brookfield Zoo, she can recall no instances in which a keeper was badly injured by an animal there.
The zoo has four polar bears, although Payton is kept alone. Katzen said he will be joined by a female bear in the near future.
The zoo's Web site calls polar bears "the most carnivorous of bears, " and says they eat "A LOT of meat." It says that when polar bears make a kill, they eat as much as they can as quickly as they can, up to 150 pounds at a time.
In January, a zookeeper in Kyoto, Japan, was seriously injured after a polar bear attacked him. In 1987, two polar bears killed an 11-year-old boy at a New York zoo after the boy climbed into their enclosure.
Katzen said polar bears-like all wild animals-can be unpredictable and dangerous. She said the zookeeper in the June 24 incident probably benefited from the fact that Payton was an adolescent polar bear and that he might have been distracted by a female polar bear in an exhibit adjacent to his.