Elon Musk on Friday asked any remaining Twitter employees who write software code to report to the 10th floor of the office in San Francisco by early afternoon, according to an email reviewed by Reuters.

The billionaire said in a follow-up email: "If possible, I would appreciate it if you could fly to SF to be present in person," adding he would be at the company's headquarters until midnight and would return Saturday morning.

The emails came a day after hundreds of Twitter employees were estimated to have decided to leave the beleaguered social media company following a Thursday deadline from Musk that staffers sign up for "long hours at high intensity."

The exodus adds to the rapid change and chaos that have marked Musk's first three weeks as Twitter's owner, during which the company's headcount had already been more than halved by layoffs and other departures to around 3,700.

Twitter told employees on Thursday that it would close its offices and cut badge access until Monday, according to two sources. Reuters could not immediately confirm whether the headquarters reopened.

On Friday afternoon, the company had started cutting off access to company systems for some of the employees who had declined to accept Musk's offer, one source told Reuters.

Another source said the company was planning to shut down one of Twitter's three main U.S. data centers, at the SMF1 facility near Sacramento, for cost-saving reasons.

In his first email to Twitter employees this month, Musk warned that Twitter may not be able to "survive the upcoming economic downturn." He also said, "We are also changing Twitter policy such that remote work is no longer allowed, unless you have a specific exception."

A White House official also weighed in, saying Twitter should tell Americans how the company was protecting their data.

Musk said Twitter has reinstated some of the controversial accounts which have been banned or suspended, including satirical website Babylon Bee and comedian Kathy Griffin, adding that a decision to bring back former U.S. President Donald Trump's account is yet to be made. When a Twitter user asked Musk to reinstate conspiracy theorist Alex Jones, Musk replied with a curt "no."

Musk tweeted a poll later on Friday asking followers to vote yes or no on reinstating Trump. The majority voted yes.

"Vox Populi, Vox Dei," Musk tweeted, a Latin phrase meaning "the voice of the people is the voice of God."

Amid the changes, Moody's withdrew its B1 credit rating for Twitter, saying it had "insufficient or otherwise inadequate information to support the maintenance of the rating."

In his emails on Friday, Musk ordered employees to email him a summary of what their software code has "achieved" in the past six months, "along with up to 10 screenshots of the most salient lines of code."

"There will be short, technical interviews that allow me to better understand the Twitter tech stack," Musk wrote in one of the emails.

Musk said earlier this week that some Tesla engineers were assisting in evaluating Twitter's engineering teams, but he said it was on a "voluntary basis" and "after hours."

He said he would try to speak with remote employees by video, and that only people who could not physically get to the company's headquarters or have a family emergency would be excused.

Musk wrote on Twitter late on Thursday that he was not worried about resignations as "the best people are staying."