The oil giant British Petroleum (BP) is to release an internal investigation report into the oil spill in the Gulf of Mexico, the worst ecological catastrophe in recent US history.The investigation has taken five months, and is expected to blame BP as well as other companies involved in the drilling operation. The report will play a key role in how BP defends itself in legal proceedings.Safety equipment on the Deepwater Horizon rig failed after an explosion on 20 April, in which 11 people died. An estimated 4.9 million barrels of oil leaked into the waters of the Gulf after the blast, with only 800,000 barrels being captured.