A day at the zoo ended in tragedy as a tiger escaped from its cage and killed a female keeper before being shot dead.

A Siberian tiger named 'Altai' escaped from its enclosure at the Cologne Zoo in western Germany on Saturday. The tiger escaped to a nearby storage building where it mauled and killed its 43-year-old keeper.  The zoo's director shot and killed the animal.

"It appears the gate wasn't properly shut," police spokesman Stefan Kirchner told The Associated Press.

"We cannot yet explain how the keeper could make such a fatal mistake (of failing to close the gate)," Pagel said, as quoted by Reuters.

Before any more damage could be done, the director used a rifle to shoot the animal through the skylight of the building. It is unlikely that any visitors of the zoo witnessed what happened.

"This is the darkest day of my life," the zoo's director, Theo Pagel, was quoted as saying by The Sun. "It appears it was a question of inadvertence."

The zoo was evacuated for a short period of time and later reopened to visitors. However a late-night opening of the zoo was cancelled.

The four-year-old tiger named Altai was previously held at a center in the UK. He fathered three cubs in November.

Animal rights group PETA said this incident is one of the reasons zoos shouldn't be in operation.

"Tragedies like this can only be prevented in the future if people stop considering as recreational fun the locking up of wild animals that have a desire for freedom," said Peter Hoeffken, a zoologist from PETA (People for the Ethical Treatment of Animals).

The Cologne Zoo in western Germany is one of the oldest and most popular attractions in the country. It houses about 10,000 animals from 700 species.