Two Somali men will spend the rest of their lives in prison for hijacking a yacht off the coast of Africa and killing the four Americans on board, a federal judge in Norfolk, Va., ruled Monday.

Muhidin Salad Omar and Mahdi Jama Mohamed pleaded guilty to killing missionaries Jean and Adam Scott along with their friends Bob Riggle and Phyllis Macay in May 2011.

The Scotts had been living on their boat, the Quest, since 2002. They were on a mission to distribute Bibles around the world when the hijacking took place.

Nineteen Somali pirates raided the vessel in hopes of bringing the Americans to Somalia to be ransomed.

Their plans went awry when they realized U.S. Navy warships were shadowing the Quest. The pirates fatally shot all four passengers.

"As the government said, it's difficult to imagine what must've been going through the minds of the four victims," U.S. District Judge Mark Davis said.

Although Omar and Mohamed weren't on board when the Americans were shot, they admitted they'd taken part in the hijacking.

"Although the defendant was not on the Quest at the time of the deaths of the American hostages, he must remain accountable for their murder as he willfully engaged in the piracy of the Quest," prosecutors wrote in the government's position on Omar's sentencing.

"The boarding of the Quest by nineteen armed and desperate men, unwilling to negotiate and intent on a ransom for the Quest and its crew, certainly set the stage for the violence and murders to follow," they concluded.

The judge agreed, handing down mandatory life sentences to the men. Both asked for forgiveness for the crime.

"A life sentence sends a strong message to anyone who chooses to engage in piracy against U.S. interests that they will face severe consequences," U.S. Attorney Neil MacBride said.