That included Exxon employees' failure to close an upstream safety valve, which could have significantly reduced the size of the spill after it was first detected.
Among other response activities are on-water skimming, subsurface wellhead operations, continued efforts to see if they can get that shut-off valve to close, and significant booming efforts underway to protect vital shoreline.
Studies of the blowout preventer's two control pods suggest that a flattish battery and a dodgy valve meant that neither was in a fit state to close off the well automatically when they should have, which BP takes as evidence of poor maintenance by Transocean.