Muhidin Salad Omar, 30, was part of a Somali and Yemeni group that hijacked the S/V Quest off the Somali coast in February and he went aboard the US guided-missile warship USS Sterett to negotiate a possible ransom.

While he was aboard the American vessel, shooting broke out on the private yacht and the four Americans were killed. The rest of the pirates were captured by US military forces and brought to Norfolk, Virginia for prosecution.

Three Somalis have been charged for allegedly shooting, kidnapping and murdering the two couples: Scott Underwood Adam, Jean Savage Adam, Phyllis Patricia Macay and Robert Campbell Riggle.

Another Somali, Mahdi Jama Mohamed, was also sentenced on Monday to life in prison. Mohamed, believed to be 23 or 24 years old, admitted to guarding the hostages but was not involved in the shootings of the Americans either.

He also helped US Navy investigators who are trying to model a new profile of pirate activity and tendencies, according to a court filing. He and Omar pleaded guilty to piracy in May.

Pirates operating off the coast of Somalia have hijacked vessels in the Indian Ocean and the Gulf of Aden, making tens of millions of dollars in ransoms by seizing ships, including oil tankers, and hostages.

Judge Michael Davis ordered life sentence terms for Omar and Mohamed, a mandatory sentence for piracy.

"Somali piracy is a scourge on the world stage, and it continues to grow more widespread and more violent," Neil MacBride, US Attorney for the Eastern District of Virginia, said in a statement.

Two more from the group are due to be sentenced on Tuesday.