>We definitely can’t open source Unreal Engine 2 or 3, because of dependencies on a large number of external closed-source middleware packages with complex licensing requirements.
>Open sourcing Unreal Engine 1 might be possible, but getting the source and dependencies into a releasable state would take a lot of cleanup effort that we just haven’t been able to find time for. I hope we can do it someday. [0]
[0] https://forums.unrealengine.com/t/unreal-engine-1/14084/6