The primary negligence will most likely be on the part of Halliburton. They were doing the actual cementing operations.
Once a well is drilled, you place casing (thick pipe) in the hole as a support. You cement the pieces of pipe together. Speculation I have heard is that at such depth (a mile under water and then 2.5 miles under the seabed) the cement is not curing. If it does not cure, it can perforate, and gas can leak in through the perfs and come back up the pipe. There is tremendous static electricity on a drilling rig. One spark...
Word is that they had actually competed cementing operations, and if so, the oddds are good that no one was paying attention to the down hole pressures. During drilling and when there is something actively going on down hole, there are guys sitting in a control booth doing absolutely nothing other than monitoring the downhole conditions - pressure, temperatures, etc. If they were between phases, there might not have been the necessary dilligence. That will all be speculation, of course, since the men who might know about that were all incinerated when the well blew. In actuality, this kind of thing has happened before during cementing operations, but the BOP (blow out preventer) worked and prevented a newsworthy story.
BP's biggest issue right now is that they had NO contingency plan whatsoever for a catastrophic event like this. The question will be - did you not consider how to shut in a well at 5000+feet subsea depth? They literally have NOTHING - no fucking clue - how to shut this well in, and neither do any of the other companies that drill deepwater. "This never happens" so no one has the tools or equipment that could handle this scenario at such depths. BP's own internal risk analysis pooh-poohed the idea of such an event occurring. They determined that the risk was so miniscule, that it was not cost effective to waste time and money to come up with a means of shutting in the well if it did happen. And they were right - to a point. This just does not happen on such a catastrophic scale. But now it has, against the odds, and they are caught with their pants down. They were drilling with fingers crossed and it didn't work this time. It could have just as easily been XOM or Shell or Chevron - it just happened to be BP. So all the investigations in the world are not going to change anything. in fact, even if they wanted to buy equipment to work at this depth, it does not exist. There has never been a need and there is no market for it - until now.
So HAL will catch a lot of the heat, and the rig workers were Transocean hands, and the company man for BP will have some 'splainin to do, but at the end of the day, it is BP that will take 90% of the blame, the PR hit, and the monetary loss.