Sanctum developers, Coffee Stain Studios, have licensed the Unreal Engine 3 for Sanctum and for unknown future projects. Sanctum was first developed as a mod for Unreal Tournament 3. The game evolved from a mod into a full blown game after Epic released the UDK, or the Unreal Development Kit, a free version of their Unreal Engine 3.
Sanctum won multiple awards in Epic’s $1 Million Intel Make Something Unreal Contest, which gave the developer public exposure and helpful attention from Epic Games. Since then, Coffee Stain and Epic have had close ties, so it makes sense that they would announce some sort of partnership, especially considering how powerful the Unreal Engine is.
For more info, check out the official press release.