NAIROBI, Kenya — Pirates in Somalia said Wednesday they are ferrying ammunition and men to the 30 hijacked vessels still under their control, and they threatened to kill more captives following the violent end to a hostage standoff that left four Americans dead.

The U.S. military said that 1

5 pirates detained after the Americans were slain Tuesday could face trial in the United States.

The military, FBI and Justice Department are working on the next steps for those pirates, said Bob Prucha, a spokesman for U.S. Central Command in Florida. The Somalis are currently being held on the aircraft carrier USS Enterprise, which is in the waters off East Africa.

A pirate aboard the hijacked yacht Quest on Tuesday fired a rocket-propelled grenade at a U.S. warship that had responded to last Friday’s hijacking. Then gunfire broke out aboard the yacht. When Navy special forces reached the Quest, they found the four American hostages had been shot and killed.

The FBI is investigating the killings of Phyllis Macay and Bob Riggle of Seattle, Washington, and Jean and Scott Adam of Marina del Rey, near Los Angeles, who had made their home aboard their 58-foot yacht Quest since December 2004.

The Adams handed out Bibles around the world, but a pirate who gave his name only as Hassan told The Associated Press on Wednesday that played no factor. He said the pirates reacted violently after the U.S. forces blocked the yacht’s path.

“We had plans to either take the hostages to the inland mountains or to move onto other hijacked ships because we knew that the U.S. Navy was serious about carrying out a rescue operation,” Hassan said. “The hostages pleaded with us not to harm them or take them to dangerous places. They cried when we captured them … and asked us to release them because they were too old and couldn’t endure captivity.”

The killings came less than a week after a Somali pirate was sentenced to more than 33 years in prison by a New York court for the 2009 hijacking of the Maersk Alabama. That hijacking ended when Navy sharpshooters killed two pirates holding the ship’s American captain.

Pirates reacted angrily to the sentencing and have since vowed that they will kill hostages before being captured during military raids and being sent to face trial.

Pirates once were believed to be disgruntled and financially motivated Somali fishermen angry that international trawlers were illegally fishing Somalia’s waters. Now criminal gangs dominate the piracy trade, and have begun systematically torturing hostages, including locking them in freezers.

“What we’re seeing is that because of the business model the pirates have adopted is so lucrative that you’re now getting organized criminal gangs involved as opposed to fishermen who just decided to have a go at piracy,” said Wing Commander Paddy O’Kennedy, spokesman for the European Union’s anti-piracy force.

“Criminal gangs are more violent than your average fisherman who’s turned to piracy,” O’Kennedy said.

A pirate in Somalia who gave his name as Adowe Osman Ali said fellow “soldiers” had ferried the reinforcements to hijacked ships in their hands on Wednesday in a bid to deter more hostage rescue attempts. He said after Tuesday’s incident, captains of hijacked ships have been ordered to tell navies not to approach or hostages would be killed.

“In the past, 20 or so soldiers used to guard every ship but now the numbers are ranging between 60 and 70 soldiers,” said Ali, a pirate in the coastal village of Gara’ad.

“We are more alert than anytime before,” he said. “In the past, we allowed the foreign navies to approach us but now we have warned them to not get nearer to us.”

Piracy has plagued the shipping industry off East Africa for years, but the violence used during the attacks — and the money demanded in ransoms — have increased in recent months. Pirates now hold some 30 ships and more than 660 hostages.

The average ransom now paid to pirates is in the $5 million range, a huge leap from only three or four years ago when it was in the hundreds of thousands of dollars, said Roger Middleton, a piracy expert at the London-based think tank Chatham House. One ransom paid last year was just shy of $10 million.

“It’s really gone up, really an enormous amount,” Middleton said. “If you think you can get a $9.5 million ransom, I suppose the logic is that you try any means possible to get there, and if that means scaring some crews and owners more, I guess that’s what you do,” he said, alluding to the recent reports of torture.

Industry experts warned Wednesday it’s too soon to say whether the Americans’ deaths will require a wholesale change in the way the shipping industry operates along with the militaries patrolling the Gulf of Aden and Indian Ocean.

It’s still not known publicly whether prompted a pirate to fire a rocket-propelled grenade at a Navy war ship, and it’s unclear whether there was an internal pirate fight or if there had been a hostage escape attempt.

“We don’t know what happened yesterday so we’re not going to make any knee-jerk decisions,” O’Kennedy said. “But our policy remains the same. Nothing is off the table. All options are open to us as a military force.”

Pirates blamed the deaths of the American hostages on the U.S. Navy, saying the pirates felt under attack.

“We warned them before that if we are attacked, there would be only dead bodies,” said a man who gave his name as Abdirahman Abdullahi Qabowsade. “We have been killed and arrested illegally before, so we can’t bear with such attacks anymore. We will respond to any future attacks aggressively.”