Two BP supervisors were charged with involuntary manslaughter for the well blowout that killed 11 workers and unleashed a huge oil spill in the Gulf of Mexico, U.S. prosecutors said Thursday.

A third BP employee was also charged with obstruction of justice for lying about how much oil was gushing out of the runaway well after the Deepwater Horizon rig sank in April 2010.

Attorney General Eric Holder cautioned that more people may yet face prosecution after announcing a deal to settle criminal charges against the British energy giant for $4 billion.

"It's an ongoing investigation," Holder told reporters.

"We'll continue to follow all credible leads and pursue any charges that are warranted."

Holder also noted that the justice department had "failed to resolve" its civil case to determine how much BP (IW 1000/4) should pay in fines under the Clean Water Act and other environmental rules.

Those fines could amount to as much as $18 billion if gross negligence is found.

"We're looking forward to the trial -- which is scheduled to begin in February of next year -- in which we intend to prove that BP was grossly negligent in causing the oil spill," Holder said.

He heralded Thursday's "historic" settlement of BP's criminal charges as a "critical step forward."