BP Plc acted with gross negligence in setting off the biggest offshore oil spill in U.S. history, a federal judge ruled, handing down a long-awaited decision that may force the energy company to pay billions of dollars more for the 2010 Gulf of Mexico disaster. U.S. District Judge Carl Barbier held a trial without a jury over who was at fault for the environmental catastrophe, which killed 11 people and spewed oil for almost three months into waters that touch the shores of five states.