Authorities said thieves who knew what they were doing broke into the Beauval Zoo Saturday night in Saint-Aignan-sur-Cher, cut through the fence surrounding the primate habitat, covered a surveillance camera and made off with seven golden lion tamarins and 10 silvery marmosets.
The suspect or suspects carried the endangered animals away in the wooden boxes in which they were sleeping in family groups, zoo director Rodolphe Delord said, according to French television station TF1.
"These are extremely rare monkeys that are part of international breeding programs. The golden lion tamarins belong to the government of Brazil. These are extremely endangered species," he said.
The thieves "knew what they were taking," Delord said. The theft constitutes a loss of about 200,000 euros (nearly $228,000), officials estimate.
Investigators are studying hundreds of hours of surveillance videos from other areas of the zoo, which is located about 140 miles south of Paris.
"Only about a dozen keepers out of nearly 500 employees in the park has access" to the primate habitat, Lt. Col. Eric Chuberre of the national police in Blois told reporters Tuesday.
Authorities are exploring the possibility that the animals were stolen at the behest of some kind of private collector or maybe taken to be sold on the black market for rare animals.
Only about 250 golden lion tamarins were counted in Brazil's Atlantic coastal rain forest in the 1970s, according to Beauval Zoo's Facebook page
. As of 2015, more than 3,000 have been reintroduced to the rainforest, thanks to international breeding programs, the zoo's page said.
The silvery marmoset, also a native of Brazil, is only found in a small strip of forest in the Amazon River basin, according to FactZoo.com.
The zoo's plea for the return of the animals has gone viral on social media, with the hashtag #RendezLesSinges, which translates to "return the monkeys."
Delord is concerned about the animals' well-being and said they must be found quickly.
"They are very difficult to feed. These are monkeys that must be handled by specialists," he said, according to TF1.