Half-Life 2, Doom 3 Assets in S.T.A.L.K.E.R.?

This week, threads have started showing up on various forums containing what appears to be evidence that GSC Game World's recently released shooter S.T.A.L.K.E.R.: Shadow of Chernobyl makes use of a number of graphical assets taken directly from commercially released games such as Valve's Half-Life 2 and id Software's Doom 3. Many forum posters are speculating that the assets were used without authorization.