The conclusion of an investigation into the chaotic firing of Sam Altman from OpenAI more than three months ago represented a resounding victory for the high-profile chief executive as he moves to reassert control of the artificial intelligence company he helped to create.

OpenAI, in a news conference on Friday, said that Mr. Altman, who returned to OpenAI just five days after he was pushed out in November, did not do anything that justified his removal and would regain the one role at the company that still eluded him: a seat on the company’s board of directors.

Mr. Altman’s ouster stunned Silicon Valley and imperiled the future of one of the tech industry’s most influential start-ups. It also called into question whether OpenAI — with or without Mr. Altman in charge — was ready to carry the banner for the tech industry’s rabid focus on artificial intelligence.

When he returned to OpenAI in November, Mr. Altman did not regain his board seat while agreeing to an investigation of his behavior and the board’s actions. Two members who voted for his removal agreed to step down; their replacements, from outside the company, oversaw the investigation by the law firm, WilmerHale. Bret Taylor, chairman of OpenAI’s board, said during the news conference that the highly anticipated report about the episode was finished, but the company did not release the report.

The company said that the law firm’s report found that OpenAI’s board acted within its broad discretion to terminate Mr. Altman, but also found that his conduct did not mandate removal.

“The special committee recommended and the full board expressed their full confidence in Mr. Altman and Mr. Brockman,” Mr. Taylor said, referring to Greg Brockman, the company president who quit in protest after Mr. Altman was removed. “We are excited and unanimous in our support for Sam and Greg.”