Uber’s head of HR has resigned after only 18 months following an investigation into how she handled allegations of racial discrimination at the taxi firm.

Liane Hornsey, the firm’s chief people officer, resigned in an email to staff on Tuesday, after an investigation into accusations from anonymous whistleblowers that she had systematically dismissed internal complaints of racial discrimination.

Hornsey was Uber’s head of human resources and one of the firm’s top spokespeople on diversity and discrimination issues. Her 18 months at the company was littered by claims of widespread issues of gender discrimination and sexual harassment.

The allegations raise questions about chief executive Dara Khosrowshahi’s efforts to change the toxic culture of the firm after he took over in August last year from Uber founder Travis Kalanick following a series of scandals.

Khosrowshahi praised Hornsey as “incredibly talented, creative, and hard-working” in an email to employees but gave no reason for her departure. Hornsey said in an email to staff that she had “been thinking about this for a while” despite her exit appearing to be out of the blue.

Hornsey did not respond to requests for comment about the investigation.

An anonymous group claiming to be “Uber employees of colour”, according to Reuters, alleged that Hornsey had used discriminatory language and made derogatory comments about Bernard Coleman, Uber’s global head of diversity and inclusion. They allege that Hornsey had denigrated and threatened former Uber executive Bozoma Saint John, who left the company in June, and was ultimately behind her departure.

Saint John and Coleman declined to comment. Saint John joined Uber from Apple in 2017 and left after a year. Coleman joined Uber from Hillary Clinton’s 2016 presidential campaign.

Law firm Gibson Dunn, which conducted the investigation, told the employees in a 15 May email that some of the allegations were substantiated, and that Khosrowshahi had been informed. A subsequent investigation was launched following further allegations.

“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,” an Uber spokesperson said.

The complaints against Hornsey come a year after Uber was embroiled in widespread allegations of gender discrimination and sexual harassment, triggering an investigation by former US attorney general Eric Holder and ultimately Kalanick’s resignation.

Uber agreed to pay $10m in March to settle a proposed class-action lawsuit alleging discrimination against more than 400 women and minorities brought by three women engineers. One of the women removed herself from the class-action and sued the company in May alleging discrimination based on gender and race.

The employees behind the latest allegations said complaints filed to Uber’s anonymous tip line often were left unresolved or were dismissed, especially if they dealt with issues of race.

They also accused the company of ignoring a board-approved recommendation by Holder that its chief diversity officer report directly to the CEO or COO.