Neuralink, one of Elon Musk’s lesser-known business ventures, is facing a federal investigation into its practices after 1,500 animals are said to have been killed in trials of the company’s brain implants.

The startup, which Musk founded in 2016, is building coin-size computer chips designed to be implanted into the human brain, with the aim of helping people with conditions like paralysis or neurological disorders.

Reuters reported on Monday that an investigation into Neuralink was underway, citing official documents and sources familiar with both the investigation and operations at the company.

The U.S. Department of Agriculture’s Inspector General reportedly opened the probe in recent months at the request of a federal prosecutor, with sources telling Reuters that officials are looking into potential violations of the Animal Welfare Act.

The Animal Welfare Act regulates the treatment of animals in research and testing settings.

Representatives for Neuralink and the Department of Agriculture did not respond to requests for comment.

Staff prompt investigation

The investigation into Neuralink comes amid a slew of internal staff complaints, according to Reuters, with employees alleging the company’s rushed animal testing is causing unnecessary suffering and death.

Those complaints include claims that pressure from Musk—who is the company’s CEO—to speed up the development of Neuralink’s technology has led to substandard experiments. Reuters reported that these failed experiments needed to be repeated, leading to more and more test animals being killed.

Current and former employees told the news agency that Musk was pushing hard for faster progress at Neuralink, telling employees in a February email: “In general, we are simply not moving fast enough. It is driving me nuts!”

That internal memo reportedly came 10 minutes after another message from Musk that included a news article about Swiss scientists whose electrical implant helped a paralyzed man to walk.

The Reuters report builds on interviews Fortune carried out in January with former Neuralink employees, who described “an enterprise marked by internal tensions and erratic management,” as well as accusations from animal rights groups of abuse including “invasive and deadly brain experiments” on monkeys. The company later admitted it had killed eight monkeys during research experiments.

Last year, Neuralink released footage of what it said showed a monkey’s intended hand movements being translated to allow it to play a computer game, and in 2020 the company said it had successfully implanted a chip in the brain of a pig named Gertrude.

What does Neuralink do?

Neuralink describes itself as a “team of exceptionally talented people” who are “creating the future of brain-computer interfaces.”

As well as developing brain implants that could help quadriplegic patients communicate via their thoughts and possibly even regain motor, sensory, or visual function, the company says it is “inventing new technologies that could expand our abilities, our community, and our world.”

Some scientists, however, argue that Musk is overhyping Neuralink’s potential.

Its brain implant is designed to work by connecting thousands of neurons in the brain, recording their activity, and translating the signals directly into an external device. This would theoretically enable people with quadriplegia to control computers and mobile devices with their thoughts.

Musk has previously described the technology as “a [hotlink]Fitbit[/hotlink] in your skull.”

Just last week Musk said he himself would sign up to be a test subject once human testing was given the regulatory go-ahead.

However, the firm’s human trials have already been delayed several times, with Musk originally hoping to begin them in 2020.

This story was originally featured on Fortune.com

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