Download the BP document at this link:
My synopsis of this document:
1. EDS (Emergency Disconnect Switch) was implemented.
2. EDS would have started hydraulic rams within the Blow Out Preventer (BOP) that would have sealed the flow of oil. This should have sealed the well in 30-50 seconds.
3. EDS failed due to BOP failure.
4. The under water ROV (Robotic Ocean Vehicle) then inspected the EDS and BOP and identified UNDOCUMENTED MODIFICATIONS TO THE HYDRAULIC CONTROL SYSTEMS.
SABOTAGE is the first answer to what happened. The BOP and/or EDS had modifications made that the control engineers knew nothing about. When they hit the emergency switch to seal the flow, the wrong hydraulic systems were activated due to these unknown modifications.
WHO? That may never be answered, but it would surely be whatever crew, company or persons had access to the BOP prior to it being lowered into the water to drill the well.