User blog comment:D.Ryan/Was Subject Delta actually in BioShock 1 ?/@comment-5154803-20120611162359/@comment-1330314-20120611173550

Actually, the Drill in Suchong's corpse is the non-detachable, suggesting the Big Daddy was taken down after he killed the doctor. As said below, Delta and Eleanor were bonded in Fontaine Futuristics and not Apollo Square. Methinks it was likely Delta's predecessor (Subject Gamma?), since the Big Daddy was only starting to bond with his LS.

There have been quite a few discussions on this subject, including here and here, with more written about it.