Actually, Transocean owns a piece, too -- it eventuated that their rig crew made some unauthorized field modifications to the controls of their blowout preventer stack and kill lines. They were not, as they say in the industry, "100% API" (American Petroleum Institute, i.e. "standard").
Halliburton may also own a sliver for selling BP a "magic" new cement formula which may have failed under testing conditions in the "Macondo" well.
Perhaps. Fact is, we don't know. And won't know until all the facts come out.
In the meantime, BP bears the ultimate responsibility. Even though, in fact, it may bear none of the blame.