WASHINGTON — A US court on Monday 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.

The life sentences to Ali Abdi Mohamed, 30, and Burhan Abdirahman Yusuf, 31, are the first handed down to pirates over the incident. Both pleaded guilty; three others face charges that could carry the death penalty.

US prosecutors vowed to make the case serve as a lesson for Somalia's pirates, who have created a virtually based on hijackings and ransom payments in the strategic waters next to their lawless and isolated homeland.

The sentences "will be heard throughout the pirate community -- and should send a clear message -- that the days of unbridled armed robbery and extortion at sea are over," said Janice Fedarcyk, assistant director-in-charge of the Federal Bureau of Investigation's New York field office.

"The only plunder these pirates earned is life behind bars," she said in a statement announcing the sentences, handed by a federal judge in the major naval hub of Norfolk, Virginia.

Christian missionaries Jean and Scott Adam were sailing their Quest yacht around the world when they were hijacked.

They were shot to death, along with their companions Bob Riggle and Phyllis Macay from Seattle, Washington, several days after being taken hostage and as negotiations were taking place with US Navy officials.