BP shares were down nearly 6% Thursday after a court found the firm grossly negligent in the 2010 Deepwater Horizon Gulf of Mexico disaster, Bloomberg reports.

Fines are likely for the firm as a result of the decision. Earlier this year, an appeals court upheld a lower-court decision allowing the U.S. government to seek a maximum fine of $18 billion if Thursday's ruling of gross negligence came down.

The London-based firm has already agreed to pay out $13.7 billion, including a $9.2 billion settlement with spill claims victims. The company announced in July it was taking a $43 billion charge to cover any further penalties.

Eleven people were killed and 16 injured in the explosion.

“BP’s conduct was reckless,” federal district judge Carl Barbier wrote in the decision in New Orleans federal court, Bloomberg said.

The precise penalties will be set following hearing scheduled for next January, the Financial Times' Ed Crooks says. BP has always denied gross negligence and is expected to appeal, he adds.

Transocean and Halliburton were also found culpable but face lower fines. Their shares both climbed on the ruling.

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