A U.S. court has sentenced two Somali pirates to life in prison for killing four American hostages after hijacking their yacht off the coast of Oman earlier this year.

Officials in Norfolk, Virginia announced the sentences Monday in the case of 30-year-old Ali Abdi Mohamed and 31-year-old Burhan Abdirahman Yusuf. The two Somalis had pled guilty to piracy, which carries a mandatory life sentence. Officials also say nine co-conspirators have also entered guilty pleas and will be sentenced in the coming weeks.

Authorities said in a statement that Mohamed and Yusuf were among 19 Somalis who willingly conspired to sail the high seas in search of a vessel to seize and hold for ransom. They say the men anticipated receiving a percentage of whatever ransom they received after paying about a third of that money to the operation's financier.

The four Americans were sailing along with an international yacht race, the Blue Water Rally, before heading off on their own. Organizers of the rally say the four were sailing toward Salallah in Oman when pirates seized their vessel, the S/V Quest.

The Americans were shot to death in February, days after they were taken hostage. This was the first time U.S. citizens had been killed by the pirate gangs operating off the Somali coast. The Americans were yacht owners Jean and Scott Adam of Marina del Ray, California, and Bob Riggle and Phyllis Macay of Seattle, Washington.

U.S. naval forces had been trying to negotiate the hostages' release when, without any warning, pirates fired a rocket-propelled grenade at a U.S. warship ((the USS Sterett). The U.S. military said the grenade missed, but sailors then heard gunfire aboard the yacht.

U.S. special forces boarded the hijacked vessel and discovered all four hostages had been shot by the pirates. Despite immediate medical care, the four Americans died of their wounds.

Pirates have made hundreds of millions of dollars hijacking ships for ransom in recent years.