Instead, the government charged Robert Kaluza and Donald Vidrine, the two highest-ranking BP supervisors on board the rig in the hours before the 2010 disaster, with 11 counts of involuntary manslaughter and 11 counts of what's known as seaman's manslaughter.

The seaman's manslaughter statute was first passed by Congress in 1838 after a string of deadly steamboat accidents. The law, which has been modified multiple times since then, was intended to hold captains, engineers and pilots responsible for deaths attributable to their conduct.

The idea behind the law was that those operating a ship have a special responsibility toward passengers in their care. As The Wall Street Journal and others have noted, prosecutors face a lower bar in proving seaman's manslaughter than they do in ordinary manslaughter cases. Manslaughter charges generally require a finding of gross negligence; seaman's manslaughter does not.

The U.S. Court of Appeals for the 5th Circuit affirmed that distinction in a 2005 case, U.S. v. O'Keefe, which could be key precedent in the BP case. The O'Keefe case resulted in the seaman's manslaughter conviction of a tugboat captain whose wife drowned in the Mississippi River after the vessel capsized. At trial, the government introduced testimony indicating that the captain had ingested cocaine on the day of the accident. Prior to deliberations, the captain's lawyer asked that the jury be instructed that the government must show the accident was a result of "gross negligence," defined as "wanton or reckless disregard for human life."

U.S. District Judge Helen Berrigan in New Orleans disagreed, ruling that the statute requires only a finding of negligence, which she defined as an "omission to perform some duty" or a "violation of some rule or standard of care, which is made to govern and control one in the discharge of some duty." The 5th Circuit upheld her ruling and the captain's conviction.

But the 5th Circuit decision left unanswered the question of whether the government can assert seaman's manslaughter against Kaluza and Vidrine at all. The two are accused of failing to alert engineers onshore that BP's Macondo well was unstable and of accepting "illogical" explanations from members of the rig crew for the warning signs. In the indictment, prosecutors allege the two men violated the seaman's manslaughter statute by engaging in "negligence" and "inattention" to their duties.

Here's what the seaman's manslaughter statute says: "Every captain, engineer, pilot or other person employed on any steamboat or vessel, by whose misconduct, negligence or inattention to his duties on such vessel the life of any person is destroyed ... shall be fined under this title or imprisoned not more than ten years, or both."

Vidrine and Kaluza were not captains or engineers of the Deepwater Horizon. The two have been described by investigators, including those who wrote the "Chief Counsel's Report" on the Deepwater disaster, as "well site leaders." According to the report, well site leaders served as BP's "eyes and ears, and made important decisions regarding the course of drilling operations." As the litigation against Vidrine and Kaluza progresses, a judge may have to decide whether "well site leaders" should fall within the "other person" category covered by the seaman's manslaughter statute.

A search of the database of legal research service Westlaw shows the statute has rarely been used in the last decade, and when it has, it has mostly been against defendants who were responsible for navigating a vessel. In the case of the convicted tugboat captain, neither the trial judge nor the 5th Circuit addressed who is covered under the statute, but Judge Berrigan emphasized the statute's purpose of holding responsible those in charge of navigation.

"In light of the unique dangers of maritime travel, the vulnerability of passengers on board such vessels and the voluntary nature of employment or ownership, it is reasonable to impose on such crews a heightened degree of care with the parallel lower threshold for criminal liability," she wrote.

It may be a long shot for Vidrine and Kaluza to argue that the seaman's manslaughter statute does not apply to them, but it's probably one worth taking. Vidrine counsel Robert Habans of Habans & Carriere and Kaluza lawyer Shaun Clarke of Gerger & Clarke both declined to comment. They have previously said their clients are innocent and have been unfairly targeted by the government.

(Reporting by Andrew Longstreth; Editing by Ted Botha and John Wallace)

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