A group of lions mauled a zookeeper to death at one of Europe’s largest big-cat parks on Wednesday after a door inside their enclosure was left unlocked, investigators in Crimea said.

The mauling occurred at the Taigan safari park on the Russian-annexed Crimean peninsula, one of the largest breeding grounds for lions in Europe and home to around 60 of the predators.

“A criminal case has been opened over the death of an employee at the Taigan lion park as a result of a predator attack,” the Moscow-installed Investigative Committee of Crimea and Sevastopol said in a statement.

The employee who had worked at the park for almost 17 years died after she “went to clean a cage with three lions, without shutting the bolt of a door between two rooms of the enclosure,” investigators said.

The park’s owner Oleg Zubkov identified the victim as chief zookeeper Leokadia Perevalova, calling the incident a “tragic” mistake.

“It is unclear how and why this happened, as the animals could not have done such a thing on their own, and apparently there were no people around,” he said in a statement on his blog.

“The employees who discovered the body, unfortunately, could no longer provide any assistance, since she had simply been torn to pieces,” he said.

On his Telegram channel, Zubkov praised Perevalova as a “valuable employee” who was the “soul of our park,” adding that “the human factor of forgetfulness played a role.”

“Unfortunately when I arrived there was nothing I could do to help. (They are) top predators and they do not forgive mistakes, he said.

A criminal case has been opened into the death of a person by workplace negligence, investigators said.

The sprawling 70-acre park was opened to visitors in 2012 on the site of a former military base. According to the park’s website, about 1 kilometer of elevated walkways allow visitors to observe lions roaming freely.

Authorities closed the zoo for a month in December 2019 after they accused Zubkov of giving the animals expired feed, an allegation he denied.

He accused Crimea’s Russian-installed authorities of trying to “bankrupt” his business and of deliberately freezing his projects after they seized power in 2014.

Crimea was annexed by Russia in 2014 but is internationally recognized as part of Ukraine.

Fatal lion attacks at zoos are extremely rare. In 2018, a lion escaped its enclosure and killed a recent college graduate at a North Carolina conservatory during a routine cleaning. Authorities were told the lion was able to enter the same space as 22-year-old Alex Black because a large ball prevented the lion’s pen from closing.

In 2013,  a lion killed a 24-year-old volunteer at a Central California animal park after it escaped from a feeding cage and attacked her while she was cleaning its enclosure area.

The Associated Press contributed to this report.