(Reuters) -OpenAI Chief Executive Sam Altman will return to the ChatGPT-maker's board along with three new directors, the world's most prominent artificial intelligence company said on Friday.

An investigation by law firm WilmerHale into the events surrounding the November firing of Altman also has concluded, and the company has created new governance rules and strengthened its conflict of interest policy. The board also said it unanimously backed Altman's leadership.

Employees, investors and OpenAI's biggest financial backer, Microsoft, had expressed shock over Altman's ouster, which was reversed within days.

The company on Friday said it was appointing new directors including Altman, Sue Desmond-Hellmann, a former CEO of the Bill and Melinda Gates Foundation, Nicole Seligman, a former president of Sony Entertainment, and Fidji Simo, CEO of Instacart.

They will join current board members Adam D'Angelo, the CEO of Quora, former U.S. Treasury Secretary Larry Summers and Chairman Bret Taylor, former co-CEO of Salesforce.

The investigation by WilmerHale found that Altman's dismissal was not the result of concerns related to OpenAI's finances, product safety or other issues.

"Instead it was a consequence of a breakdown in the relationship and loss of trust between the prior Board and Mr. Altman," OpenAI said, describing the law firm's findings.

"WilmerHale found that the prior Board believed at the time that its actions would mitigate internal management challenges and did not anticipate that its actions would destabilize the Company," OpenAI said in a blogpost.

"WilmerHale found that the prior Board acted within its broad discretion to terminate Mr. Altman, but also found that his conduct did not mandate removal," it added.

CONFLICT OF INTEREST

OpenAI said it was adopting new corporate governance guidelines and creating a whistleblower hotline. The startup, whose CEO has been a prolific investor in other companies, also said it was strengthening its conflict of interest policy.

The board gave few details about those improvements.

The board's lack of detail for its surprise November decision fueled speculation about potential misconduct by Altman, which he and the company have denied, and about supposed existential risks from the technology that OpenAI is building.

Altman's return as CEO about four days after his firing came after nearly all of OpenAI's employees threatened to depart unless the board restored Altman and resigned.

His return led to discussions about how OpenAI would be governed, and the company announced a reconstituted board that did not include Altman and was helmed by Taylor.