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The tan macaque with the hairless pink face could do little more than sit and shiver as her brain began to swell. The California National Primate Center staff observing her via livestream knew the signs. Whatever had been done had left her with a “severe neurological defect,” and it was time to put the monkey to sleep. But the client protested; the Neuralink scientist whose experiment left the 7-year-old monkey’s brain mutilated wanted to wait another day. And so they did.

As the attending staff sat back and observed, the monkey seized and vomited. Her pupils reacted less and less to the light. Her right leg went limp, and she could no longer support the weight of her 15-pound body without gripping the bars of her cage. One attendant moved a heat lamp beside her to try to stop her shaking. Sometimes she would wake and scratch at her throat, retching and gasping for air, before collapsing again, exhausted.

An autopsy would later reveal that the mounting pressure inside her skull had deformed and ruptured her brain. A toxic adhesive around the Neuralink implant bolted to her skull had leaked internally. The resulting inflammation had caused painful pressure on a part of the brain producing cerebrospinal fluid, the slick, translucent substance in which the brain sits normally buoyant. The hind quarter of her brain visibly poked out of the base of her skull.

On September 13, 2018, she was euthanized, records obtained by WIRED show. This episode, regulators later acknowledged, was a violation of the US Animal Welfare Act; a federal law meant to set minimally acceptable standards for the handling, housing, and feeding of research animals. There would be no consequences, however. Between 2016 and 2021, the United States Department of Agriculture (USDA) enforced the humane treatment of animals through what it called “teachable moments.” Because the center—home to a colony of nearly 5,000 primates run by the University of California–Davis—had proactively reported the violation, it could not be legally cited.

And neither could Neuralink. “If you want to split hairs,” a former employee tells WIRED, “the implant itself did not cause death. We sacrificed her to end her suffering.” The employee, who signed a confidentiality agreement, asked not to be identified.