Certainly not a hoax. The army began an investigation into these abuses last January (i.e., very quickly after the abuses themselves occurred). Here's an excerpt from a New Yorker article describing the investigation:
"A month later, General Karpinski was formally admonished and quietly suspended, and a major investigation into the Army’s prison system, authorized by Lieutenant General Ricardo S. Sanchez, the senior commander in Iraq, was under way. A fifty-three-page report, obtained by The New Yorker, written by Major General Antonio M. Taguba and not meant for public release, was completed in late February. Its conclusions about the institutional failures of the Army prison system were devastating. Specifically, Taguba found that between October and December of 2003 there were numerous instances of “sadistic, blatant, and wanton criminal abuses” at Abu Ghraib."