The US Food and Drug Administration has rejected clinical trials of billionaire Elon Musk's Neuralink brain implant, seven current and former employees told Reuters on Thursday. The regulator cited multiple safety concerns the company must address before human testing can begin – a milestone they had hoped to reach this month.

The FDA cited dozens of "deficiencies" barring the way to human trials when it rejected Neuralink last year, the employees explained. Dozens of pigs have already been used in experimental service to address the agency's concern that the tiny threads that interface with the wearer's brain could migrate, altering brain and implant functionality, causing inflammation, rupturing blood vessels and otherwise damaging the fragile tissue, employees said.

The FDA is also reportedly concerned about whether Neuralink can be removed without causing damage – an issue the employees claimed has not been adequately addressed, despite the company's assurances.

Just like the lithium-ion batteries that power Musk's Tesla electric vehicles, which can burn for hours at 3,000 degrees Fahrenheit if struck the wrong way, Neuralink's lithium batteries could cause brain damage if they so much as overheat, experts told Reuters. The FDA wants Neuralink to prove with animal studies that the remotely-chargeable battery is "very unlikely" to fail.

Displeased with the regulatory red-light, Musk has reportedly rushed the animal experiments in an attempt to address the FDA's safety concerns. As a result, the Department of Agriculture is looking into potential violations of the Animal Welfare Act after alarms were raised about the treatment of the animals. Meanwhile, the Department of Transportation is investigating whether Neuralink correctly followed safety procedures when disposing of chips removed from animal brains.