New Orleans ( CNN ) -- A titanic courtroom showdown with billions of dollars in the balance opened in New Orleans on Monday , with oil giant BP arguing it should n't face the government 's steepest penalties for the 2010 Gulf oil spill . BP already pleaded guilty to criminal charges and agreed to a record-setting $ 4 billion fine for the spill . But it could face more than $ 20 billion in additional environmental penalties if found to have committed gross negligence in the disaster . In a packed federal courtroom Monday afternoon , BP attorney Mike Brock said blame for the disaster was n't the oil company 's alone . A string of bad decisions by Transocean , the company that owned the doomed drill rig Deepwater Horizon ; well cement contractor Halliburton ; and BP all led to the blowout , he said . `` We do not believe that men and women of BP behaved in willful misconduct , '' Brock said . `` It was a multiparty event . '' But Halliburton attorney Don Godwin said BP ignored the contractor 's recommendations about the cement job and that Transocean did n't move fast enough to contain the blowout . Transocean settled with the government last week for $ 1 billion in Clean Water Act penalties but could face additional additional liability in the case that started Monday . Transocean lawyer Brad Brian said that last week 's settlement was not an admission of gross negligence and that last-minute changes to the well design by BP had the rig 's crew `` at wits ' end '' before the disaster . All three companies have been pointing fingers at each other since the April 20 , 2010 , blowout that sank the Deepwater Horizon , killed 11 men aboard and uncapped an undersea gusher that spewed for nearly three months . The spill 's effects on the environment are still being cataloged . The plaintiffs in the civil case that opened Monday include five Gulf states , individuals , businesses and the federal government . `` Evidence will show BP placed huge financial pressure to cut costs , cut corners , and rush the job , '' attorney James P. Roy , who represents the coalition of plaintiffs , said during opening arguments on Monday . And Alabama Attorney General Luther Strange said the oil giant `` was blinded by their bottom line . '' `` The spill was tragically inevitable due to BP 's corporate culture , '' Strange said . `` The evidence will show that , at BP , money mattered most . '' If it is found to have been `` grossly negligent '' under the Clean Water Act , it could be fined as much as $ 4,300 per barrel of oil spilled into the Gulf of Mexico . If it 's found `` negligent , '' the company could be fined about $ 1,100 per barrel . Attorneys will also likely square off over how much oil gushed into the gulf , another key figure that will be used to calculate how much money BP might owe . Officials have said 4.9 million barrels -- about 205 million gallons -- of oil spilled , while BP says that number is overblown and that authorities should use a maximum figure of 3.1 million barrels of oil when calculating the fine . The trial will also determine what fines the company faces under National Resource Damage Assessment , which aims to restore environmental damage caused by the spill . Environmental groups want to see those fines -- which will put a specific price tag on damage to plants and wildlife -- total around $ 25 billion . With so much money at stake , each side has brought an army of lawyers to the fight . With almost 60 lawyers filling the courtroom , the judge created a seating chart for all the attorneys . Lawyers representing the federal government and other plaintiffs sat on one side , while the BP lawyers and other companies ' lawyers sat across the room . Three overflow courtrooms were also packed on Monday . BP says it has already paid billions in spill-related cleanup and compensation costs and has been barred from new federal contracts . Though Halliburton and Transocean could also face penalties , much of the criticism from environmentalists before the high-profile trial has focused on BP , the undersea well 's owner . `` The damage done here is real , both to the environment and to the people , '' said Brian Moore , of the National Audubon Society . `` And BP should not have the chance to get off cheaply on this . '' In his statement before the trial began , BP General Counsel Rupert Bondy said the company would push for the court to consider lower penalties , arguing that BP made efforts to do the right thing and `` immediately stepped up '' and acknowledged its role in the spill . `` To date we 've spent more than $ 23 billion in response , cleanup , and payments on claims by individuals , businesses and governments , '' he said . `` No company has done more , faster , to meet its commitment to economic and environmental restoration efforts in the wake of an industrial accident . '' CNN 's Vivian Kuo and Catherine E. Shoichet contributed to this report .
NEW : The Gulf oil spill was `` a multiparty event , '' BP lawyer argues . Billions of dollars are at stake as a judge weighs negligence and financial penalties . BP is fighting to limit fines , while the company 's critics hope to see a landmark punishment . Score: 6