The 8-year-old patas monkey, named Kyle, died in February, and the incident is already spurring change at the zoo.
Zoo officials say they changed procedures after Kyle's death, but animal rights supporters say the death didn't have to happen.
The zoo rebuild a home for Kyle, but the monkey died before he could see it.
"Clearly what went down was despicable," said Nicole Meyer with In Defense of Animals. "The fact that an animal died at the hands of zoo staff -- I think that's just egregious."
Meyer is upset about the death, and the group's complaint prompted a federal inquiry into Kyle's transfer out of his older exhibit.
Zoo staff spent five months training for the transfer, yet it still ended in Kyle's death.
At the time, zoo officials said Kyle would often pick medication out of his food and that tranquilizer darts weren't an option. The situation was already delicate because Kyle had lost an arm due to a previous bone infection.
In the end, zoo staff decided using a net was the best way to go.
"If being an expert means handling an animal that leads to the animal's severe injuries and ultimate death, then I question what the term 'expert' means," Meyer said.
Back in March, zoo officials said Kyle put up a struggle during the transfer and wound up with a broken leg. He was later euthanized.
In a written statement, zoo officials maintain the March account and confirm they did receive a warning from the US Department of Agriculture in the aftermath of Kyle's death.
"The zoo is reviewing and revising its animal transfer protocol the zoo had no prior warnings or incidents on record with the USDA," the statment reads.
If there are any more violations, the zoo could face civil penalties or other sanctions. The zoo has plans to bring in two new female monkeys soon.