Somali pirates have killed four Americans aboard their yacht in the Indian Ocean, news reports said Tuesday.

Efforts by the United States Navy to negotiate the release of the American captives were underway when they were shot dead, according to the reports. The S/V Quest had been hijacked last Friday off the coast of Oman.

U.S. forces said they heard gunfire aboard the yacht, and when they later boarded they found the passengers dead. That differs from what the pirates recounted. They said U.S. forces fired first, killing two of the pirates, and the pirates killed the hostages in retaliation, according to the BBC.

The U.S. Navy captured 13 pirates, killed two, and found two other pirates dead on the yacht.

The American passengers were identified as Scott and Jean Adam, Phyllis Mackay and Bob Riggle. In 2004, the Adams had embarked on a trip around the world for eight to 10 years.

Piracy in the Indian Ocean has flourished as more Somalis resort to crime as their nation continues to be mired in civil war and lawlessness without the administration of a central government.