Developer Undead Labs is currently working on a "pure survival" sandbox mode for its zombie game State of Decay, which recently passed half a million paid downloads, says executive producer Jeff Strain.

According to Xbox Wire, State of Decay is the fastest-selling original game ever published through Xbox Live Arcade, selling 500,000 copies since its launch on June 5.

Strain promised Undead Labs will continue to push updates for State of the Decay based on fan request. The second title update for the game will add a new mode designed to embody the zombie-survival experience.

"For a start, I can tell you we're working on a sandbox mode designed to be a pure survival simulation," Strain said.

Eurogamer reports that a follow-up game to State of Decay, a massively multiplayer online title currently known as Class4, is currently in limbo awaiting Microsoft's approval.

"We cannot begin working on Class4 without a greenlight from Microsoft (who owns the IP, as part of the bargain that led to the funding and development of State of Decay) and a mutually agreeable contract," director of community Sanya Weathers told Eurogamer. "We have to agree on where we think the technology is going to be in a few years (since the game will take a few years to develop); we have to agree that we have the same goals; and we have to agree on how we'll collectively run and support the final game.

"Those discussions are still in progress," she added.

Strain also revealed that Undead Labs is currently working on the Windows PC version of State of Decay, which will launch through Steam.