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Californian as part of an investigation into claims of gender bias at Riot Games.

The studio is behind the hugely popular online game League of Legends.

The California Department of Fair Employment and Housing (DFEH) is seeking evidence that Riot is paying women less than men.

DFEH said it was also looking into accusations of sexual harassment and assault at the firm as well as discrimination on pay and promotion.

The "enforcement action" filed by DFEH against Riot aims to force the game-maker to hand over information about employee pay.

So far, said a statement by the DFEH, Riot had "refused" to give the regulator enough information to work out whether men were paid more while working at the studio.

In a statement given to the Verge news site, to the DFEH's requests and had provided more than 2,500 pages of documents and "several thousand lines of pay data".

It added: "We've been in active conversations with the DFEH since its inquiry began. Investigations like this can arise when there have been allegations of workplace disparity and we've been co-operating in good faith with the DFEH to address its concerns."

It said it was "disappointed" to see the DFEH claim it was not co-operating with the investigation.

News about the investigation comes after a mass walkout in May organised to protest about the way Riot handled claims of gender bias.

About 150 employees walked out after Riot took legal action to stop two women suing the company over discrimination. The lawsuit attempted to stop the pair taking legal action and forced them to enter arbitration talks.

Riot claimed the contract of employment signed by the women meant they had waived their right to sue.

At the time of the walkout, Riot said it planned to change its internal policies so employees did not lose the right to sue and were not forced into arbitration.