Prey is less talked about than Duke Nukem Forever, but it may have had an even longer and more tortuous development process. First kicked off by 3D Realms in 1995, the game's development was initially led by Tom Hall of Doom and Deus Ex fame, and its engine was developed in-house by William Scarboro. However, Hall along with most of the original Prey team quit the project one year later in 1996, leaving Scarboro and a new team to pick up the pieces over the following months.
By 1997, the new team was showing off Scarboro's innovative "portal technology," which allowed areas inside the game world to seamlessly connect to each other regardless of their location. Development continued until late 1998, when both Scarboro and the new project lead suddenly left 3D Realms with little official explanation. A new tech lead was hired by 3D Realms to write another in-house engine, but she disappeared from the project in early 2000, and Prey's development was handed over to Human Head Studios in 2001. Human Head started development over from scratch using id Software's Doom 3 engine, but retained many of the design elements introduced by the previous teams—including the intra-level portals—and finally drove the game to completion.