Jean and Scott Adam (PICTURES): Americans slain by Somali pirates were delivering Bibles

(CBS/KIRO/AP) Jean and Scott Adam were on a mission to distribute Bibles around the world when their work tragically ended after their yacht was hijacked by Somali pirates and they were murdered on Monday.

PICTURES: 4 Americans on hijacked yacht dead off Somalia

The killings mark the first time U.S. citizens have been killed in a wave of pirate attacks plaguing the Gulf of Aden and the Indian Ocean for years.

The yacht named Quest was the home of the California couple who had been sailing around the world since December 2004 with a yacht full of Bibles. Two other Americans on board were also killed, Phyllis Macay and Bob Riggle, of Seattle, Wash.

U.S. naval forces, who were trailing the Americans' captured yacht with four warships, quickly boarded the vessel after hearing the gunfire and tried to provide lifesaving care to the Americans, but they died of their wounds, the U.S. Central Command said in a statement from Tampa, Fla.

Two pirates died during the confrontation and 13 were captured and detained, the U.S. Central Command said. The remains of two other pirates who were already dead for some time were also found. The U.S. military didn't state how those two might have died.

On Monday, two pirates had peacefully come aboard the USS Sterett to negotiate with naval forces for the release of the hostages, and remained aboard overnight.

But on Tuesday, pirates aboard the hijacked Quest unexpectedly fired a rocket-propelled grenade at the Sterett. Shortly afterward, gunfire erupted inside the Quest cabin, and U.S. special forces responded, approaching the Quest in small boats and boarding the vessel, Vice Adm. Mark Fox, commander of U.S. Naval Forces Central Command said in a Tuesday press conference.

According to CBS affiliate KIRO, Scott Adam, in his mid-60s, had been an associate producer in Hollywood when he turned in a spiritual direction and enrolled in the seminary a decade ago, said Robert K. Johnston, a professor at Fuller Theological Seminary in Pasadena and a friend of Adams.

"He decided he could take his pension, and he wanted to serve God and humankind," he said.

Johnston and Adam worked together to start a film and theology institute. Adam also taught a class on church and media at the school.

Since 2004, the Adams lived on their yacht in Marina Del Rey for about half the year and the rest of the year they sailed around the world, often distributing Bibles in remote parts of the Fiji Islands, Alaska, New Zealand, Central America and French Polynesia, Johnston said.

Scott and Jean Adam documented their maritime missionary work on their website, S/V Quest Adventure Log.

"We express our deepest condolences for the innocent lives callously lost aboard the Quest," said Gen. James N. Mattis, U.S. Central Command Commander.

Johnston said that despite an adventurous spirit, the Adams were meticulous planners who knew the dangers they faced. The couple had sailed with a large flotilla to stay safe from pirates near Thailand earlier in the trip.