(CNN) -- Before two pirate leaders departed the captured yacht where they held four Americans earlier this month, a maritime source says they left instructions: Kill the hostages if we do not come back from negotiations.

U.S. officials later took the two negotiating pirates into custody -- a move that goes against standard negotiation practices, the maritime source said.

The four Americans were later killed, but it is not clear why.

Also not clear is when during the negotiations -- or why -- the Americans detained the two pirate negotiators.

The pirates' detention goes against standard negotiating practices, as the pirates came in good faith to make a deal to hand over the hostages, said the maritime source, who was briefed on the incident and has connections to British intelligence officials.

The source asked not to be identified because of the sensitivity of the situation.

U.S. Central Command declined to comment on whether officials detained the pirates and said the FBI and the Justice Department have the lead in the case. The Justice Department had no comment Sunday. On Friday, the FBI said there was no comment due to the continuing investigation.

However, a U.S. government official told CNN the talks with the two pirate negotiators faltered when it was determined the men "had no authority to negotiate anything." American officials then took the men into custody rather then let them return to the yacht, the U.S. official said.

"It would be unfathomable to have put them back on the boat where they might have done harm to the hostages," the official said, disagreeing with a published report that the detention of two fellow pirates might have upset those still aboard the captured yacht -- named the Quest -- who were holding four Americans.

"There was no change in mood, no escalation" and gunfire broke out on the Quest more than eight hours later, the U.S. official said.

The official took issue with a New York Times report that said the FBI's hostage negotiator aboard the USS Sterett decided the two Somalis were not serious about resolving the matter, which led U.S. officials to take the two men into custody and ask for a new representative from the pirates.

According to the the U.S. official, who was not authorized to discuss the Quest hijacking on the record, the issue was not whether the two pirates were serious about negotiating, but that they lacked the power to cut a deal.

The American side continued trying to negotiate via radio with those still aboard the yacht and made an offer, and were told by the suspects aboard the Quest they would sleep on the matter before providing a reply, the U.S. official said. The Navy also tried to contact Somalis on land who might be able to exert influence over the men holding the Quest.

The FBI hostage negotiator is part of a special team based at Quantico, Virginia, and had field experience, the U.S. official said. It was unclear if that experience included any previous negotiations with Somali pirates.

Americans Jean and Scott Adam, Phyllis Macay and Bob Riggle were found shot to death after U.S. forces boarded the Quest around 1 a.m. Tuesday, U.S. officials have said.

The 58-foot yacht was being shadowed at the time by the military after pirates took it over off the coast of Oman on February 18.

U.S. forces had responded after a rocket-propelled grenade was fired at a U.S. Navy ship about 600 yards away -- and missed -- and the sound of gunfire could be heard on board the Quest, U.S. Navy Vice Admiral Mark Fox has said. The killings took place as negotiations involving the FBI were under way for the hostages' release.

When Fox spoke last week, he said two pirates boarded a U.S. Navy ship Monday for talks. He told reporters he had no information on details of the negotiations or whether a ransom had been offered.

Two pirates were found dead on board the Quest, said Fox. In the process of clearing the vessel, U.S. forces killed two others, he said. Thirteen other pirates were captured and detained on the Quest, along with the other two already aboard the U.S. Navy ship.

Fox said authorities believe the pirates were trying to get the vessel and hostages to Somalia, or at least into Somali territorial waters.

Piracy has flourished recently off the coast of Somalia, which has not had an effective government for two decades.

Globally, more than 50 pirate attacks have already taken place in 2011. As of February 15 -- the most recent statistic posted on the International Maritime Bureau's website, pirates were holding 33 vessels and 712 hostages.

CNN's Zain Verjee and Carol Cratty contributed to this report