(NewsNation) — Officials at the Dallas Zoo believe two monkeys were stolen after they were found missing from their enclosure Monday morning.

Officials at the Dallas Zoological Park said the emperor tamarin monkeys’ enclosure had been “intentionally compromised” and that a search of the zoo grounds did not turn up the missing mammals.

“Based on the Dallas Police Department’s initial assessment, they have reason to believe the tamarins were taken,” the zoo said on Twitter.

The disappearance comes after zoo officials said earlier this month they found a cut in the monkey habitat enclosure. A clouded leopard also went missing at the zoo this month and had cuts in its habitat, as well. The monkey and leopard enclosures are next to each other, according to a map of the zoo.

The zoo was closed Monday and isn’t expected to reopen until Thursday, the Dallas Morning News reported.

Aside from the missing animals, an endangered vulture was found dead in a case deemed “unusual” by zoo officials.

Tim Harrison, an exotic pet specialist, previously told NewsNation these types of potential crimes can sometimes point to a handful of potential culprits.

“ALF, the Animal Liberation Front, will sometimes come in and do these things as you know … we call them basically eco-terrorists,” Harrison said.

On its website, People for the Ethical Treatment of Animals says the two groups have “very different approaches” but does “not condemn (ALF) for carrying out illegal actions in which no sentient being is harmed.”

ALF recently took credit for a mass release of more than 10,000 minks from a farm in Van Wert County, Ohio in December.