BP said approximately 2.5 million barrels of oil were spilled over the course of 87 days. An accident at the Deepwater Horizon oil rig left 11 workers dead and led to one of the worst oil spills in the history of the industry.
A four-week civil trial in New Orleans could add another $18 billion in penalties to BP, on top of the $42.4 billion it already has set aside for cleaning up the southern U.S. coast affected by the spill.
The federal government estimates about 4.1 million barrels of oil spilled before responders managed to cap the leak. Larry Schweiger, president and CEO of the National Wildlife Federation, said he questioned BP's figures.
"We should be extremely skeptical of BP's current claims about the volume of oil released into the gulf," he said in a statement Monday. "Getting the estimate reduced may boost the company's bottom line [in terms of penalties], but it will hurt the prospects for restoration in the Gulf of Mexico."
The U.S. government last year enacted the so-called RESTORE Act, which funnels civil penalties paid by parties responsible for the spill to gulf restoration activity.