(Bloomberg) -- Federal prosecutors dropped manslaughter charges against BP Plc’s two top employees on the doomed oil rig that blew up in 2010, the latest setback for investigators probing the largest offshore oil spill in U.S. history.

The dismissal of the most serious charges against Robert Kaluza and Donald Vidrine, who supervised testing on the Macondo well, means the two won’t stand trial for the deaths of 11 men killed in the explosion aboard the Deepwater Horizon. Kaluza, 65, and Vidrine, 68, were among four BP workers to face charges in the aftermath of the spill.

The U.S. accused the pair of ignoring multiple signals that the well was dangerously unstable before the explosion. Kaluza and Vidrine were each charged with 11 counts of involuntary manslaughter and one count of violating the U.S. Clean Water Act.

Vidrine pleaded guilty Wednesday to the pollution count. Kaluza will continue to fight the remaining charge, said his lawyer Shaun Clarke. Trial on the final count begins Feb. 16 in New Orleans.

The two BP well-site managers had earlier won dismissal of 11 counts of seaman’s manslaughter, a separate crime, after a court ruled that the victims weren’t seamen.

VP Acquitted

David Rainey, BP’s former vice president of exploration, won acquittal in June of a federal charge that he lied to prosecutors in connection with company estimates of the size of spill. Kurt Mix, a former BP engineer, pleaded guilty in November to a misdemeanor of deleting text messages related to spill estimates, following reversal of an earlier conviction.

The Justice Department dropped the involuntary manslaughter charges against Kaluza and Vidrine after deciding it couldn’t prove its case against the men, agency spokesman Peter Carr said Wednesday.

Federal prosecutors re-evaluated charges against Kaluza and Vidrine after hearing testimony in the civil trials over the causes of the spill. “A number of witnesses have testified multiple times since the indictment was brought, and the factual landscape has changed,” Carr said in an e-mail.

Legal Standard

The department couldn’t meet the legal standard for involuntary manslaughter, defined as “wanton or reckless disregard for human life,” he said. The U.S. believed it could justify seaman’s manslaughter charges, but an appeals court refused to reinstate those counts.

Vidrine agreed to perform 100 hours of community service, complete 10 months of probation, and pay $50,000 in restitution, Carr said. The judge must still approve the deal, whose terms can’t be altered, Carr said. A single count of violating the U.S. Clean Water Act is punishable by a fine and as long as a year in jail.

Vidrine, who was BP’s night-shift company man aboard the Deepwater Horizon drilling rig, said in a statement accompanying his plea that he was aware of abnormal pressure readings during key tests conducted before the explosion to determine whether the well was safely sealed against leaks. These readings indicated hydrocarbons might be seeping into the well, he agreed in the statement. Hydrocarbons rising from the well reached an ignition source on the rig, triggering the explosion and fire.

The well-site managers were singled out as scapegoats for failures by multiple individuals and companies involved in the Macondo drilling operation, Clarke, Kaluza’s lawyer, said at their initial arraignment in November 2012.

‘Nothing Has Changed’

“Nothing has changed for Bob Kaluza since he was arraigned, when he said he was innocent and that he’d trust his fate to the judge and jury,” Clarke said Wednesday in a phone interview. “As long as there’s a criminal charge out there, he intends to completely clear his name at trial.”

BP agreed in 2012 to plead guilty to 14 charges, including 11 of manslaughter, and pay $4 billion to resolve all criminal claims against it arising from the Deepwater Horizon blowout.

The company reached a $20.8 billion settlement this year with the U.S. and five states to settle civil pollution claims. BP has set aside a total of $55 billion to pay for the disaster.

The case is U.S. v. Kaluza, 12-cr-00265, U.S. District Court, Eastern District of Louisiana (New Orleans).

To contact the reporters on this story: Margaret Cronin Fisk in Detroit at mcfisk@bloomberg.net; Laurel Brubaker Calkins in Houston To contact the editors responsible for this story: Michael Hytha at mhytha@bloomberg.net Andrew Dunn.

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