Two monkeys are believed to have been taken from Dallas Zoo following a string of bizarre incidents at the attraction.
Dallas Police said no arrests had been made in any of the investigations – which also included cages being cut and the suspicious death of an endangered bird – and did not confirm whether the incidents were linked.
In the latest case, officers believe someone cut an opening in an enclosure and took the two emperor tamarin monkeys, which are small primates with long whiskers that resemble a moustache.
“It was clear the habitat had been intentionally compromised,” the zoo said in a statement.
Earlier this month, the zoo closed for a day-long search when a clouded leopard named Nova went missing.
She was eventually found near her habitat, but police said a tool had been used to cut an opening in its fencing.
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A similar opening was found in an enclosure of langur monkeys, though none got out.
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A week later, an endangered vulture called Pin was found dead, with the zoo saying the death did not appear to be natural.
Gregg Hudson, the zoo’s chief executive, said the vulture was found with a wound.
Ed Hansen, chief executive of the American Association of Zoo Keepers, said he could not recall a zoo that had dealt with so many similar incidents with such frequency.
“It appears that somebody really has an issue with the Dallas Zoo,” he said.
Mr Hansen described Dallas Zoo’s reputation as “excellent” within the industry, adding that accredited zoos had double-perimeter fencing and that a zoo as large as Dallas’ would have a security patrol.
Dallas Zoo said in a statement that it had added additional cameras and increased overnight security patrols.
It also said the two missing emperor tamarin monkeys would most likely stay near their habitat, but that a search of the zoo grounds had failed to find them.
Animals have previously escaped enclosures from Dallas Zoo, with a 340-pound (154kg) gorilla named Jabari jumping over a wall in 2004.
He went on a 40-minute rampage that injured three people before being shot and killed by police.
