MECHELEN, Belgium (AP) — A lion on the loose forced the evacuation of one Belgium's most famous zoos Thursday, and the animal had to be killed after several failed attempts to sedate it.

Planckendael zoo was evacuated for several hours in peak school-trip season as security tried to get the female lion back in its enclosure.

When it got close to an area with visitors, the decision was made to shoot it.

"It became too dangerous and after the advice we got from the police, the crisis team decided to eliminate the animal," said zoo director Dries Herpoelaert.

Until then, no visitor or staff member had been in danger.

The lion got out just after opening time so few people were at the zoo, some 30 kilometers (19 miles) north of the Belgian capital, Brussels.

"This is terrible and inexplicable," said regional animal welfare minister Ben Weyts. "I do not understand why not more attempts were made to sedate the lioness."

Weyts called for an investigation into both the escape of the animal and the decision to kill it.

Herpoelaert said it was a gut-wrenching decision: "This is a nightmare for every zoo director. A dangerous animal escapes" with the risk of people getting harmed.

"We love those animals, it's a tragedy," he said.

In the end, he said there was no choice.

"For three hours we tried to get the animal back into its enclosure" before several attempts to sedate it also failed, Herpoelaert said.