(CNN) -- A female zookeeper died Sunday morning when she was attacked by wolves in an enclosure at the Kolmarden Zoo in Sweden.
The zookeeper was a longtime employee at the wildlife park and had worked with the group of wolves since they were born, the zoo said in a statement on its website.
The attack happened during a routine visit to the wolves' enclosure, designed to "maintain contact with the wolves."
The keeper, who was not identified, followed procedure and informed her colleagues before entering the enclosure.
"When the co-worker did not respond when called, her colleagues realized something was wrong and went to the wolves' area," the zoo statement said. "There they found her dead."
It's not known what preceded the attack.
Mats Olson, the zoo's chief executive, said the staff is "shocked" by the incident and an investigation is under way.
On its website, Kolmarden -- billed as Scandinavia's largest wildlife park, and located about 150 kilometers (about 93 miles) southwest of Stockholm -- touts visitors' ability to have "wild encounters" with its animals, including petting wolves. The statement Sunday said the particular enclosure involved in the attack "lies beyond the public areas of Kolmarden."
Mats Hoggren, the zoological chief, said all procedures at the zoo will be reviewed.
CNN's Per Nyberg contributed to this report.