Echon's right, Bioware was the original developer and copyright holder of the Infinity Engine code. (It was subject to certain licenses, like the underlying D&D ruleset, but the code itself was theirs.) Interplay published the IE games that Bioware wrote, and Interplay licensed the engine for Planescape and the IWD games.
I believe at some point over the past ten years Hasbro ended up owning the engine, however, because Bioware's copyright was disappearing from re-releases of the games.