SAN FRANCISCO - Uber's Chief People Officer Liane Hornsey resigned in an email to staff on Tuesday, following an investigation into how she handled allegations of racial discrimination at the ride-hailing firm.

The resignation comes after Reuters contacted Uber on Monday about the previously unreported investigation into accusations from anonymous whistleblowers that Hornsey had systematically dismissed internal complaints of racial discrimination.

Hornsey is head of Uber's human resources department and one of the firm's top spokespeople on diversity and discrimination issues. She had been in the role for about 18 months.

Chief Executive Dara Khosrowshahi praised her in an email to employees, which was seen by Reuters, as "incredibly talented, creative, and hard-working." He gave no reason for her departure.

Hornsey acknowledged in a separate email to her team at Uber, also seen by Reuters, that her exit "comes a little out of the blue for some of you, but I have been thinking about this for a while."

She also gave no reason for her resignation and has not responded to requests for comment about the investigation.

The allegations against her and Uber's human resources department more broadly were made by an anonymous group that claims to be Uber employees of color, members of the group told Reuters.

They alleged Hornsey had used discriminatory language and made derogatory comments about Uber Global Head of Diversity and Inclusion Bernard Coleman, and had denigrated and threatened former Uber executive Bozoma Saint John, who left the company in June.

They also said complaints filed to Uber’s anonymous tip line were often left unresolved or were dismissed, especially if they dealt with issues of race.

The complainants also accused the company of ignoring a board-approved recommendation by former U.S Attorney General Eric Holder that its head of diversity report directly to the company's CEO or COO.

Uber told Reuters in a statement that the complaints had been properly investigated.

"We are confident that the investigation was conducted in an unbiased, thorough and credible manner, and that the conclusions of the investigation were addressed appropriately," it said.