Hours after new “Chief Twit” Elon Musk issued an “extremely hardcore” ultimatum to work hard or get out, thousands of Twitter employees have beelined for the exits, with the social media company disabling badge access and shuttering its brick-and-mortar spaces for at least the next 72 hours, according to multiple Thursday night reports.

The devolution into chaos began after Musk sent an early-morning internal email suggesting Twitter’s employees prepare to “long hours at hard intensity” in order to build “Twitter 2.0” on Wednesday. Musk imposed a deadline of 5 p.m. Eastern time by which staff had to report their decision to stay or go on a company Google form. By the time the deadline rolled around, according to Fortune reporter Kylie Robison, as much as three-quarters of the remaining workforce had tendered their resignations.

The exact number of staffers who survived Musk’s cull earlier this month—and chose not to quit in the intervening days—is unclear, but estimates hovered close to 3,000 workers just prior to the deadline, according to The Verge’s Alex Heath.

Several “critical” engineering teams resigned in their entirety on Thursday, according to Heath. “You cannot run Twitter without this team,” one current engineer told him, speaking in reference to one of the groups.

Also on Thursday night, according to the managing editor of Platformer, Twitter “alerted employees that effective immediately, all office buildings are temporarily closed and badge access is suspended.” No elaboration on a motive was publicly issued, according to the tech blog’s Zoë Schiffer, but she went on to tweet that sources had told her new owner Elon Musk and his team “are terrified employees are going to sabotage the company.”

Twitter’s offices are expected to reopen to whatever is left of its workforce on Monday, Schiffer reported. Platformer’s report was corroborated by Fortune and The Verge soon after.

Twitter’s employees numbered around 7,500 when Musk was handed the reins at the beginning of November. Almost immediately, he set about laying off about half of the work force. Several top executives unceremoniously quit soon after, including an exodus of its chief cybersecurity officer and four more high-level officials on Nov. 10.

“It feels like all the people who made this place incredible are leaving,” one Twitter staffer told The Verge on Thursday. “It will be extremely hard for Twitter to recover from here, no matter how hardcore the people who remain try to be.”

This is a breaking story and will be updated.