A lion escaped its den Tuesday at Woodland Park Zoo, though zoo staff said employees and visitors were never in any danger.

According to a news release from the zoo, the 12-year-old female African lion escaped into a service hallway at the felines building shortly before the zoo closed. Zoo and veterinary staff armed themselves with tranquilizer darts and sedated the lion just before 5 p.m.

The zoo's two other lions -- a 12-year-old male and a 2-year-old female -- remained in their dens during the incident, according to the zoo's release.

It isn't known how the lion managed to escape. The zoo said it will conduct an investigation into the incident.