Editor's note: This story has been updated to include new information.

Oil giant BP pleaded guilty Thursday to criminal charges for its role in the 2010 Gulf of Mexico oil spill, reaching a $4.5 billion settlement with the U.S. government. That's the largest such fine in American history, dwarfing the previous record of $2.3 billion paid by Pfizer in 2009 for medical marketing fraud.

And as the Justice Department hinted last year, three BP employees have also been indicted with manslaughter in connection with the spill, which killed 11 people and leaked 5 million barrels of oil into the Gulf. Unsealed Thursday afternoon, the indictment accuses two well-site leaders of failing to report problems on the Deepwater Horizon rig, according to the Associated Press. A BP executive is also charged with lying to investigators about how much oil was leaking from the mile-deep well.

The settlement means BP is pleading guilty to 11 felony charges of misconduct or neglect, Reuters reports, as well as two misdemeanors — one under the Clean Water Act, one under the Migratory Bird Treaty Act — and one felony count of obstruction.

"All of us at BP deeply regret the tragic loss of life caused by the Deepwater Horizon accident as well as the impact of the spill on the Gulf coast region," BP chief executive Robert Dudley said in a statement. "We apologize for our role in the accident, and as today's resolution with the U.S. government further reflects, we have accepted responsibility for our actions."

BP has already agreed to pay $20 billion into a trust fund for Gulf Coast rehab, and reported last month that it has paid $19.1 billion so far, with a final $860 million expected this quarter. The company also reached a $7.8 billion settlement in March with private-sector plaintiffs, and faces a civil trial in New Orleans early next year.

But that wasn't enough for the Justice Department, which has accused BP of gross negligence and a "culture of corporate recklessness" that allowed the catastrophic spill to occur. The plea deal resolves all U.S. criminal and securities claims, BP explains, but it won't cover federal civil claims, private civil claims, private securities claims, state economic-loss claims, or federal and state natural-resources claims.

By pleading guilty to the 12 felony counts and two misdemeanors, BP has agreed to pay the Justice Department $4 billion over five years, and the Securities and Exchange Commission $525 million over three years. The company adds, however, that it "is prepared to vigorously defend itself against remaining civil claims."

U.S. Attorney General Eric Holder formally announced the settlement at a news conference in New Orleans Thursday afternoon, noting that it "marks the largest single criminal fine and the largest total criminal resolution in the history of the United States." It's also not the end of the story, he added, explaining that the criminal investigation is ongoing. He also said next year's civil lawsuit will not be affected.

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Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

BP to pay historic penalty for 2010 oil spill
The oil giant has pleaded guilty to 12 felony counts and will pay a record $4.5 billion fine. Three of its employees have also been charged with manslaughter.