Zoo employees discovered the black and orange speckled lizards – indigenous to Sonora Desert on the US-Mexican border – were missing early on Sunday morning.

“Someone specifically broke a window near the cashier booth to enter the terrarium nearby,” zoo director Theo Pagel told Cologne-based daily Express. “They knew what they were doing and apparently knew exactly what they were stealing.”

According to Pagel, the poisonous little beasts could fetch several thousand euros each on the black market. Police in Cologne are asking for anyone with information concerning the theft to contact them.

The zoo has since reopened the building where the break-in occurred, but the reptile house will remain closed until they are sure the thieves swiped all of the Gila Monsters from the terrarium.

“We don’t know if the robbers took all of the animals or if one of them got away and is hiding,” the head of the zoo section Thomas Ziegler said.

Gila Monsters – one of two types of poisonous lizards on the planet – can be deadly to humans, but they normally only bite when they feel threatened. Instead of injecting venom through fangs like snakes, the chubby lizards grab hold of their victims with a ferocious snap and ooze poison into the wound from their lower jaw.