San Francisco: Tech billionaire Elon Musk's brain-chip company Neuralink is reportedly facing a legal challenge from an animal rights group that has accused the company of subjecting monkeys to "extreme suffering" during years of gruesome experiments. The organisation is accusing Neuralink and UC Davis of nine violations of the federal Animal Welfare Act -- a federal measure designed to reduce suffering during animal experiments, reports The New York Post. Neuralink's brain chips were implanted in monkeys' brains during a series of tests at the University of California, Davis from 2017 to 2020, according to a compliant from the Physicians Committee for Responsible Medicine filed with the US Department of Agriculture. In one example, a monkey was allegedly found missing some of its fingers and toes "possibly from self-mutilation or some other unspecified trauma". Also ReadTesla manufacturing in China & selling in India not digestible: Gadkari The monkey was later killed during a "terminal procedure," the group said in a copy of the complaint shared with The Post. In another case, a monkey had holes drilled in its skull and electrodes implanted into its brain, then allegedly developed a bloody skin infection and had to be euthanized, according to the complaint. In a third instance, a female macaque monkey had electrodes implanted into its brain, then was overcome with vomiting, retching and gasping. Days later, researchers wrote that the animal "appeared to collapse from exhaustion/fatigue" and was subsequently euthanized. An autopsy then showed the monkey had suffered from a brain hemorrhage, according to the report. The experiments involved 23 monkeys in all. At least 15 of them died or were euthanized by 2020, according to the group, which based the report on records released through California's open records law. Musk claims that Neuralink's brain chips will one day make humans hyper-intelligent and let paralyzed people walk again.