The video game development company behind the "Half-Life" series, Valve, was founded in 1996 in Kirkland, Washington by former Microsoft employees Mike Harrington and Gabe Newell. Valve began working on the first game of the series soon after the company's formation, and settled on a concept for a horror-themed 3D action game, using the "Quake" engine as licensed by id Software. The game was a hit at the 1997 E3 convention, where its animation system and artificial intelligence were demonstrated. The game's success led to its first expansion pack, "Half-Life: Opposing Force", which was developed by Gearbox Software, a then-new company based in Plano, Texas, and announced on April 15, 1999. Gearbox founder Randy Pitchford said in an interview that he believed Valve gave them the opportunity to produce a sequel to "Half-Life" to allow Valve to focus on future games. The game was demonstrated at the 1999 E3 convention, where new locations, characters, and the story were revealed.