Hundreds of employees at Twitter are estimated to be resigning from the company following an ultimatum from its owner Elon Musk that staffers sign up for long hours at high intensity or quit.

Company officials reportedly told employees that all buildings were being temporarily closed, effective immediately, and that their ability to re-enter the office with ID access was being suspended for the time being, according to two people familiar with the company’s action. It is still unclear how many exactly how many people left Twitter but the employees cited fear of professional retaliation while sharing their plans to quit.

“Entire teams representing critical infrastructure are voluntarily departing the company, leaving the company at serious risk of being able to recover. We are skilled professionals with lots of options, so Elon has given us no reasons to stay and many to leave,” an engineer was quoted as saying by a news agency. In a companywide email, Elon Musk asked employees to brace for long hours at high intensity adding that for Twitter to succeed “we will need to be extremely hardcore.”

NEW: The designers leading Elon Musk’s Blue verified project are out, along with the lead web engineer. Many Twitter employees who maintained critical infrastructure have resigned. This is going to look like a very different company tomorrow.

— Zoë Schiffer (@ZoeSchiffer) November 18, 2022