There were multiple control mechanisms – procedures and equipment – in place that should have prevented this accident or reduced the impact of the spill, the company said.

The investigation is focused on seven mechanisms including: the cement that seals the reservoir from the well; the casing system, which seals the well bore; the pressure tests to confirm the well is sealed; the execution of procedures to detect and control hydrocarbons in the well, including the use of the blow out preventer (BOP); and the BOP emergency disconnect system, which can be activated by pushing a button at multiple locations on the rig.

The mechanisms also include the automatic closure of the BOP after its connection is lost with the rig; and features in the BOP to allow remotely operated vehicles (ROV) to close the BOP and thereby seal the well at the seabed after a blow out.

The investigation has not reached final conclusions, but has identified various issues for further inquiry. BP said that it has shared these early perspectives with the Department of the Interior and will do so with all official regulatory inquiries into the accident as requested.

According to the company, this is an internal investigation. There is further work to do – including further interviews, and in addition full forensic examinations of the BOP, the wellhead and the rig itself – all of which are still currently on the sea bed. The internal investigation was launched on April 21, 2010.

Tony Hayward, chief executive of BP, said: “I understand people want a simple answer about why this happened and who is to blame. The honest truth is that this is a complex accident, caused by an unprecedented combination of failures. A number of companies are involved, including BP, and it is simply too early – and not up to us – to say who is at fault.

“This is a basic summary of the facts as gathered by the investigation team to date. A lot remains unknown, but we hope that the briefings will help the government’s inquiries. This was a tragic accident and we need to understand the causes of it to try to ensure that nothing like it ever happens again.”