In 2017, the billionaire Tesla CEO claimed in an article on Tim Urban’s blog Wait But Why that within four years, his new, under-the-radar startup called Neuralink would have a device on the market that would not only translate thoughts into actions on computers, but also help cure “certain severe brain injuries.” Then, on July 16, 2019, Musk proudly revealed Neuralink’s tech to the public in a San Francisco launch event, promising that human trials would begin in 2020. 

Those trials didn’t happen, but last year, at a Nov. 30 “Show and Tell” event, Musk assured the crowd that Neuralink would gain approval for its tech by the spring. “I could have a Neuralink device implanted right now, and you wouldn’t even know,” he joked.

That won’t be happening either. Owing to safety concerns, the U.S. Food and Drug Administration (FDA) just rejected Neuralink’s late 2022 application to begin human trials, Reuters reported Thursday, citing seven unnamed current and former employees.

The FDA’s key concerns include questions about how Neuralink’s technology can be implanted or removed without damaging the brain as well as issues with the use of lithium batteries—which are a known fire hazard. The regulator also warned about the potential for the wires that Neuralink uses to pass electrical signals from the brain to the computer—called Links—to have unintended effects on movement or feeling. 

It’s a major setback for Musk, but not the end of his brain-computer interface dreams, as the FDA characterized the approval process as an ongoing conversation, Reuters reported. It’s also not an unforeseen setback. Tim Harris, a senior fellow at Janelia Research Campus and the lead designer of a BCI startup called Neuropixels, told Fortune’s Jeremy Kahn and Jonathan Vanian last year that Musk would need to implant hundreds of Links in each user to make his device work properly.

“It is a major worry from my perspective,” Harris said. “How many holes can you insert and still feel?”

Still, just last year, the BCI startup Synchron, after gaining FDA approval in 2021, became the first company to implant a chip into a human’s brain with the goal of enabling people with severe paralysis to control computers. Microsoft cofounder Bill Gates and Amazon’s Jeff Bezos are both investors in the company.

Musk founded Neuralink in 2016, and has said that, in the future, people will be able to use his company’s technology to “replay memories” and argued it will even enable someone with paralysis to use a smartphone faster than able-bodied people.

In order to accomplish this task Musk brought his famous hard-nosed culture to the startup, leading the majority of the scientists he started the project with to leave the firm. A half-dozen former employees told Fortune last year that they were under immense pressure to meet unrealistic deadlines. 

“There was this top-down dissatisfaction with the pace of progress even though we were moving at unprecedented speeds,” one former employee said. 

Tim Hanson, a scientist at the Janelia Research Campus who was part of Neuralink’s founding team, said the issue is a “mismatch” between the pace at which basic science can develop and what Musk and Neuralink want done.

“Everyone in that whole empire is just driven by fear,” another employee added.

Still, Neuralink investors remain confident that the company’s technology will be revolutionary. The startup has received over $360 million in funding since its inception, with backing from high-profile investors like Google Ventures, according to Crunchbase data.

“I definitely would never bet against him,” Bob Nelsen, a cofounder of ARCH Venture Partners who has invested his own money into Neuralink, told Reuters this week. “If he has some bumps in the road with Neuralink, or any other thing, he’ll regroup and figure it out.”