U.S. Customs and Border Protection officers at the Progreso International Bridge made an unusual discovery last week when they found four endangered spider monkeys hidden inside a duffel bag.

According to a news release, on Dec. 30, a 20-year-old woman arrived at the Progreso International Bridge in a 2017 Jeep Patriot, and a CBP officer referred her for further inspection. 

While conducting the secondary inspection, officers discovered four spider monkeys concealed inside a duffel bag. Working together with CBP agriculture specialists, the officers removed the monkeys from the bag and placed them in an animal container.

“While conducting their inspections, our officers will often encounter a myriad of prohibited agriculture products,” Port Director Walter Weaver said in the news release. “Sometimes these encounters yield hidden exotic animals, such as in this case.”

READ ALSO: Tiger Cub Rescued by Border Patrol Now on Display at Brownsville Zoo  

The U.S. Fish and Wildlife Service (USFWS) identified the animals as spider monkeys, considered endangered under the Endangered Species Act.

Officials said the woman was issued a penalty by CBP, and the spider monkeys were returned to Mexico.

The incident is a familiar occurrence to workers at the Gladys Porter Zoo, where zookeepers offer rehab to distressed or illegally obtained wildlife. 

Zoo Director Dr. Patrick Burchfield says the spider monkeys intercepted could have been headed for the wildlife pet trade, with more at risk.

"That tells me, if we're going to see a pulse of spider monkeys, they're probably deforesting an area of tropical deciduous or tropical rainforest for probably slash and burn agriculture or maybe some big development," Dr. Burchfield said. 

It's not the first time exotic animals were recovered in the Valley. A tiger cub was found along the river by Border Patrol agents in 2018. 

"It costs us an excess of $100,000 a year just for the animals we rehabilitate, and that's not a budgeted item," said Dr. Burchfield. "So, it really stretches an already stretched budget pretty thin."