BP lied to the U.S. government and withheld information about the amount of oil spilling into the Gulf of Mexico after its well blew out in 2010, attorneys told a judge Monday.

But lawyers for the London-based oil giant denied those accusations and said there was no way to prepare for such a unique blowout a mile below the sea floor. Second-guessing the company’s efforts to cap the well was “Monday morning quarterbacking at its worst,” BP attorney Mike Brock said during opening statements of the second phase of a trial over the spill.

This part focuses on BP’s response to the disaster and is designed to help U.S. District Judge Carl Barbier determine how much oil spewed into the Gulf.

The government’s estimate is 70 million gallons more than what BP says spilled. Establishing how much oil leaked into the Gulf during BP’s struggle to cap the well will help figure out the penalties the oil company must pay. Billions of dollars are at stake.

The first phase of the trial centered on what caused the blowout.

Brian Barr, an attorney for residents and businesses who claim they were hurt by the spill, said BP failed to prepare for a blowout and compounded the problem by misleading federal officials.

BP had a 600-page oil spill response plan that only included one page on “source control.” It simply called for assembling a team of experts to devise a way to stop a blowout, Barr said.

“BP’s plan was nothing more than a plan to plan,” he said.

The April 20, 2010, blowout of BP’s Macondo well 50 miles off the Louisiana coast triggered an explosion that killed 11 workers on the Deepwater Horizon drilling rig and spawned the nation’s worst offshore oil spill. BP used a capping stack to seal the well on July 15, 2010, after other methods failed.

BP maintains its spill preparations complied with every government requirement and met industry standards. But the blowout presented unforeseen challenges, Brock said.

In May 2010, BP tried in vain to use the “top kill” method to stop the flow of oil by pumping mud and other material into the blowout preventer. Plaintiffs’ lawyers claim BP knew the strategy was doomed to fail based on higher flow rate estimates that the company didn’t share with federal officials at the time.

“Nevertheless, BP pressed ahead and falsely claimed that it was a slam dunk,” said Brad Brian, an attorney for rig owner Transocean.

In videotaped testimony shown in court Monday, former Energy Secretary Steven Chu said BP didn’t provide him with certain data that could have been relevant in evaluating the “top kill” procedure.

“If one knew earlier the amount of flow, things would have been different,” Chu said.

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A week after the spill started, high-ranking BP official Doug Suttles told Coast Guard Rear Adm. Mary Landry that the company estimated oil was flowing at a rate of 1,000 to 5,000 barrels per day. But the company’s adversaries at the trial say BP’s own internal documents and emails show an effort to conceal much higher estimates.

“BP’s policy of not releasing the flow-rate information was enforced at the highest levels of the company,” Brian said, pointing to an email that a BP employee sent to BP Exploration and Production CEO Andy Inglis and his assistant on May 15.

The employee, Mike Mason, warned them that they should be “very cautious” standing behind the lower estimate because his team’s models showed estimates that were up to 20 times higher.

Mason said he was called in for a meeting the next day with Inglis’ assistant, who suggested he shouldn’t have put his warning in writing. When Mason asked what the problem was, the assistant replied, “It’s the big number,” Mason recalled in videotaped testimony.