pbr BP's investigation into the Gulf of Mexico spill sparked a backlash from its contractors and US politicians who accused the oil giant of attempting to shift some of the blame.br ppbr The internal inquiry, led by the company's head of safety and operations, Mark Bly, found BP was responsible in part for the tragedy, but also pointed the finger at rig owner Transocean and cement contractor Halliburton.br ppbr The report said a "complex and interlinked" series of events involving mechanical failures and human judgments led to the disaster.br ppbr Swiss-based company Transocean attacked the BP investigation in a strongly worded statement.br ppbr It said: "This is a self-serving report that attempts to conceal the critical factor that set the stage for the Macondo incident: BP's fatally flawed well design. In both its design and construction, BP made a series of cost-saving decisions that increased risk - in some cases, severely."br ppbr Halliburton claimed there were "substantial omissions and inaccuracies" in BP's report.br ppbr The explosion on April 20 killed 11 workers and caused an estimated 4.9 million barrels of oil to gush into the Gulf - the largest offshore spill in history.br ppbr BP has already spent £5.2 billion trying to contain the disaster, and has forecast that it will eventually cost the group more than £20.9 billion, after clean-up costs and compensation are taken into account.