Early last year, a hacker gained access to the internal messaging systems of OpenAI, the maker of ChatGPT, and stole details about the design of the company’s A.I. technologies.

The hacker lifted details from discussions in an online forum where employees talked about OpenAI’s latest technologies, according to two people familiar with the incident, but did not get into the systems where the company houses and builds its artificial intelligence.

OpenAI executives revealed the incident to employees during an all-hands meeting at the company’s San Francisco offices in April 2023 and informed its board of directors, according to the two people, who discussed sensitive information about the company on the condition of anonymity.

But the executives decided not to share the news publicly because no information about customers or partners had been stolen, the two people said. The executives did not consider the incident a threat to national security because they believed the hacker was a private individual with no known ties to a foreign government. The company did not inform the F.B.I. or anyone else in law enforcement.

For some OpenAI employees, the news raised fears that foreign adversaries such as China could steal A.I. technology that — while now mostly a work and research tool — could eventually endanger U.S. national security. It also led to questions about how seriously OpenAI was treating security, and exposed fractures inside the company about the risks of artificial intelligence.

After the breach, Leopold Aschenbrenner, an OpenAI technical program manager focused on ensuring that future A.I. technologies do not cause serious harm, sent a memo to OpenAI’s board of directors, arguing that the company was not doing enough to prevent the Chinese government and other foreign adversaries from stealing its secrets.