A Guide to the Complex Gulf Oil Spill Case Monday marked the start of a potentially monthslong trial to determine the cause of a well blowout that triggered a massive oil spill and killed 11 oil workers in the Gulf of Mexico nearly three years ago. The trial also will assign blame for the spill. A glance at the history of the spill, the players involved, and what's at stake in the trial: The spill began after a well in the Gulf of Mexico owned by London-based oil giant BP PLC blew out on April 20, 2010, triggering an explosion of the Deepwater Horizon rig that killed 11 workers and spewed what the federal government estimates at about 200 million gallons of oil. Much of it spilled into the Gulf. The spill fouled marshes, killed wildlife and closed vast areas to fishing. Scientists warn that the disaster's full effect on the Gulf food chain may not be known for years. They have reported dying coral reefs and fish afflicted with lesions and illnesses that might be oil-related. Last March, BP reached a settlement with lawyers representing Gulf Coast businesses and residents who claim the spill cost them money. In December, BP reached a settlement with the Justice Department to plead guilty to manslaughter and other charges and pay $4 billion in criminal penalties. Last month, the owner of the Deepwater Horizon rig, Transocean Ltd., reached a separate settlement with the government pleading guilty to a misdemeanor charge and agreeing to pay $1.4 billion in criminal and civil penalties. The federal government and Gulf states haven't resolved civil claims against the company that could be worth more than $20 billion. That highest figure is calculated on the amount of oil the federal government estimates spilled from the well. The government estimates 4.9 million barrels, or about 200 million gallons of oil, spilled. BP says it was 4.1 million barrels, or about 170 million gallons. The maximum fine the company can be charged is about $4,300 per barrel, bringing the total maximum fine to $21 billion. The trial has been divided into two phases: The first will determine what caused the blowout and divide the blame between BP, Transocean and Halliburton, and the second will look at what efforts the companies made to stop the spill. It is not until the second phase that U.S. District Judge Carl Barbier will rule on how much more money the companies owe. The ruling could come months from now. Attorneys for the federal government: BP put profits ahead of safety and deserves most of the blame for the disaster. The catastrophe resulted from BP's "culture of corporate recklessness," and gross negligence. Attorneys for BP: We will refute charges of gross negligence. The government has inflated the amount of oil that leaked from the well and into the Gulf. BP is not blameless, but other companies that worked on the ill-fated drilling project made crucial mistakes.