OpenAI’s announcement, which was published in a blog post on the company website, did not provide details about Altman’s alleged lack of candor. The company said that CTO Mira Murati would replace Altman as interim CEO effective immediately, while a search for a permanent replacement begins.

“Mr. Altman’s departure follows a deliberative review process by the board, which concluded that he was not consistently candid in his communications with the board, hindering its ability to exercise its responsibilities. The board no longer has confidence in his ability to continue leading OpenAI,” the blog post read.

OpenAI President Greg Brockman announced he was resigning hours later, apparently in solidarity with Altman. “Based on today’s news, I quit” he wrote in a note to his employees, which he posted on X. OpenAI had stripped Brockman of his Chairman title as part of the leadership change announcement, though the company said he would remain in his operational role.

Altman, 38, said on X that his time working at OpenAI had been “transformative” and that he “will have more to say about what’s next later.”

The surprise news of Altman’s ouster and the company’s vague insinuation of some kind of deceitfulness on his part spread across Silicon Valley’s tech community like wildfire. OpenAI singlehandedly ignited the industry-wide fascination with AI almost exactly one year ago when it released its ChatGPT product and Altman has been the public face of the generative AI craze ever since.

Microsoft, which has invested $13 billion in OpenAI and incorporated its GPT technology across its product line, saw its shares plunge 1.7% following the announcement on Friday, wiping out nearly $47 billion in market value.

“We have a long-term agreement with OpenAI with full access to everything we need to deliver on our innovation agenda and an exciting product roadmap,” Microsoft CEO Satya Nadella tweeted on Friday afternoon, adding that the company is “committed to our partnership, and to Mira and the team. Together, we will continue to deliver the meaningful benefits of this technology to the world.”

Microsoft would not comment on OpenAI’s claim that Altman had not been consistently candid with OpenAI’s board. Speculation about the nature of Altman’s actions was rife on X.

Altman’s departure appears to have happened unexpectedly. Just last week Altman presided over OpenAI’s first-ever developer conference in San Francisco, rolling out new AI tools and an updated version of its GPT-4 large language model. And on Wednesday evening, Altman made an appearance at the Cerebral Valley AI conference’s networking session and speaker dinner in San Francisco. Eric Newcomer, the event organizer, wrote in his newsletter on Friday that Altman “was well-dressed and seemed in good spirits.”

“Theories abound,” about the cause of Altman’s departure, Newcomer wrote. “Everyone wants to know how much money OpenAI has really been burning.”

Semafor’s Reed Albergotti reported Friday that Altman had been in the process of raising a VC fund that was separate from OpenAI and focused on “hard tech” before his departure, though he said it was unclear if there was any connection.

Christie Hemm Klok for Fortune

Longtime tech journalist Kara Swisher said that “more major departures” within the company were imminent, and she pointed to OpenAI’s unusual structure as potentially being related to the shakeup. “As I understand it, it was a ‘misalignment’ of the profit versus nonprofit adherents at the company,” Swisher tweeted.

OpenAI began as a non-profit lab focused on academic research, but later created a for-profit arm in order to finance the increasingly expensive job of training neural networks and hiring computer science talent. The structure is all the more unusual in that the for-profit arm caps investors’ returns at a multiple of their initial investment.

OpenAI was valued at $29 billion in April when it raised $300 million in venture capital. According to a September report by the Wall Street Journal, the company is currently exploring a tender offer of some of it shares that would value the company between $80 billion and $90 billion, making OpenAI the most valuable privately-held startup in the U.S.

Tech moguls are voicing support for Altman

Several prominent tech industry figures spoke out in support of Altman on Friday, including Airbnb CEO Brian Chesky who tweeted that Altman is “one of the best founders of his generation and has made an immense contribution to our industry.”

Former Google CEO Eric Schmidt tweeted that “Sam Altman is a hero of mine. He built a company from nothing to $90 Billion in value, and changed our collective world forever. I can’t wait to see what he does next.”

Altman cofounded OpenAI in 2015, along with Brockman, Ilya Sutskever, Elon Musk and several other Silicon Valley power players. Musk left the company’s board in 2018 citing potential for conflicts of interest with Tesla and its autonomous driving efforts.

Before taking the top job at OpenAI, Altman led Silicon Valley’s legendary startup incubator Y Combinator. Unlike some tech industry leaders known for their engineering chops, Altman rose to prominence for his ability to raise large sums of venture capital and for his bets on ambitious technology such as nuclear fusion and quantum computing.

One of the founding goals of OpenAI is to create artificial general intelligence, or AGI, the idea of a single AI system capable of matching or exceeding humans in all economically-valuable cognitive tasks.

“If AGI is successfully created, this technology could help us elevate humanity by increasing abundance, turbocharging the global economy, and aiding in the discovery of new scientific knowledge that changes the limits of possibility,” Altman wrote in February.

Read more about OpenAI and Sam Altman:

• Mira Murati, the young CTO of OpenAI, is building ChatGPT and shaping your future
• The inside story of ChatGPT: How OpenAI founder Sam Altman built the world’s hottest technology with billions from Microsoft

Do you have insight to share? Got a tip? Contact Kylie Robison at kylie.robison@fortune.com, through secure messaging app Signal at 415-735-6829, or via Twitter DM.