In an act that zoo officials have described as incomprehensible, a Siberian tiger fatally attacked its keeper in front of onlookers at Germany’s Münster Zoo on Thursday. The zookeeper, a 56-year-old man identified only as Martin H., was killed instantly in the attack after reportedly failing to lock a door inside of the enclosure.

According to the German edition of The Local, the attack took place on Thursday afternoon during a scheduled feeding. The zookeeper had laid out food for the zoo’s tigers in an area outside of their compound, but had apparently forgotten to make sure that the latch on the door to their pen was secured. That’s when, officials say, 10-year-old Rasputin, crept out of the enclosure and snuck up behind Martin without attracting his attention, fatally biting him in the neck.

Although visitors at the zoo watched as the attack unfolded and called for help, rescue workers arrived too late. Jörg Adler, chief of the Münster Zoo, told reporters at a press conference on Thursday evening that at that point, there was nothing anyone could do to save Martin’s life. “An encounter like that with a tiger isn't something that can be survived,” he said. Dieter Knaack, 55, a spokesman for the Münster fire service, which responded to the emergency call at 4:25 p.m. said: “He [Rasputin] sprang on his back as fast as lightning from behind and killed him with a bite on his neck. The zookeeper had no chance. He died immediately.”

“There were witnesses, which means we do a reconstruction of the keeper's death,” Adler said, noting that Martin had been a very experienced keeper. “We can install only so much technology and so many alarms -- when it comes down to it it's the keeper who decides.”

The zoo has decided not to put Rasputin down as a result of the attack, reasoning that Martin’s death was most likely caused by human error. However, Adler told reporters that he still wondered what if anything he could have done differently.

“It is a tragedy which is hard to comprehend. I cannot describe it,” Adler said.

The attack is not the first or even second of its kind this year. In January, a Siberian tiger mauled and badly injured a man who lept into its enclosure at the Bronx Zoo. In March, another Siberian tiger at Quebec’s Lac-St-Jean zoo attacked its zookeeper, although the worker did not sustain life-threatening injuries. In 2011, a Siberian tiger in Cologne also attacked and killed a zookeeper who is also believed to have forgotten to shut its door.

Animal rights activists in Germany are reportedly outraged by the attack, complaining that it’s yet another example of why tigers should not be held in captivity. The activist group Fier Pfoten told Westdeutsche Allgemeine Zeitung that the animals are “incredibly challenging to keep in captivity and potentially extremely dangerous not only for keepers but also for visitors.”