A federal appeals court in Virginia is weighing the conviction of a Somali man called the biggest U.S. catch in the international war on piracy in waters off Africa.

Judges engaged in a lively back-and-forth Tuesday with an attorney for Mohammad Saaili Shibin and a government prosecutor.

Shibin was sentenced last August in Norfolk to a dozen life sentences for his role in two brutal hijackings off Africa. One involved the slayings of four Americans on their yacht by pirates in 2010.

The appeal, being considered by 4th U.S. Circuit Court of Appeals, hinges on the definition of piracy. Shibin’s attorney says he was not part of the high seas attack so he could not be convicted of piracy.

The government argues Shibin’s actions are defined as piracy under international law.