Two tiny endangered monkeys froze to death last week at a zoo in central Louisiana.

Three cotton-top tamarins were left outside at the Alexandria Zoological Park when a caretaker "overlooked" them, according to CBS News. The small marmosets weigh less than a pound each and require a temperature of 76 to 85 degrees. When overnight temperatures dropped to the teens last Wednesday in Alexandria, two of the monkeys died from exposure.

City spokeswoman Cynthia Jardon could not provide details about the ongoing investigation, according to CBS News, so it isn't clear if the monkeys were forgotten completely or if they had a warm nesting area available to them. The caretaker resigned on Monday, Jardon told CBS News.

There are only 1,800 cotton-top tamarins remaining in captivity and 6,000 in northwest Colombia, so the population is considered critically endangered, according to the International Union for Conservation of Nature.