Most of these former employees requested anonymity, concerned about violating nondisclosure agreements and the possibility of drawing Musk’s ire. Musk and Neuralink did not respond to multiple requests for comment for this story.

Musk has put the startup under unrelenting pressure to meet unrealistic timelines, these former employees say. “There was this top-down dissatisfaction with the pace of progress even though we were moving at unprecedented speeds,” says one former member of Neuralink’s technical staff, who worked at the company in 2019. “Still Elon was not satisfied.” Multiple staffers say company policy, dictated by Musk, forbade employees from faulting outside suppliers or vendors for a delay; the person who managed that relationship had to take responsibility for missed deadlines, even those outside their control.

Employees were constantly anxious about angering Musk by not meeting his ambitious schedules, former employees said. “Everyone in that whole empire is just driven by fear,” another former employee says, referring to Musk’s businesses, including Neuralink. This culture of blame and fear, former employees said, contributed to a high rate of turnover. Of the eight scientists Musk brought in to help establish Neuralink with him, only two, Dongjin Seo and Paul Merolla, remain at the company.

The pressure could be particularly problematic because of the multiple tough scientific and engineering challenges Neuralink was tackling. Tim Hanson, a scientist at the Janelia Research Campus that is part of the Howard Hughes Medical Institute in Ashburn, Va., was part of Neuralink’s founding team, working on its surgical robot as well as animal studies using brain-computer interfaces. “There’s this mismatch,” he says, between the speed at which engineering obstacles can be solved and the more deliberative pace of fundamental science. “Basic science is basically slow,” says Hanson, who left the company in 2018. 

Engineers sometimes had to make decisions about issues such as electrode design before relevant data was available from scientific teams working on animal research. Animal research can take months and years; the engineers were under pressure to act in days and weeks. There were also delays caused by Neuralink’s need to fabricate custom-designed computer chips, one former employee said. Musk, meanwhile, wanted to move into human implantation as fast as possible.

Tensions of this sort aren’t atypical for startups working at the cutting edge of a new technology, and similar pressure from Musk has been chronicled at his other companies, including SpaceX and Tesla. But the issues certainly add to doubts about whether Neuralink will be able to live up to the hype Musk’s has created with his breezy pronouncements about what its technology will be able to do. For more on Neuralink, and what it has and hasn’t managed to achieve, read Fortune’s full story here.