This undated combination of photos provided Tuesday Feb. 22, 2011 by the Del Rey Yacht Club, shows Scott, left, and Jean Adam of Marina del Rey, Calif. The couple were killed by pirates Tuesday after being taken hostage several hundred miles south of Oman. (AP Photo/Del Rey Yacht Club)

JASON STRAZIUSO, Associated Press

MALKHADIR M. MUHUMED, Associated Press

NAIROBI, Kenya - A U.S. military spokesman said Wednesday it was possible that 15 pirates detained after the killing of four American yacht enthusiasts could be sent to the United States to face trial.

Pirates in Somalia, meanwhile, said they were on heightened alert Wednesday after the violent end to the American hostage standoff. Pirates said they ferried more ammunition and fighters on board hijacked ships in their hands, and they threatened to kill hostages if international warships approached.

The military, FBI and Justice Department are working on the next steps for the 15 pirates detained Tuesday, 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.

Prucha couldn't say whether the FBI had yet interviewed the 15 suspects.

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 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.

That could represent a serious change from the time when pirates were believed to be disgruntled and financially motivated Somali fishermen angry that international trawlers were illegally fishing Somalia's waters.

This undated photo provided by the Del Rey Yacht Club shows the Quest, a sailboat piloted by Scott and Jean Adam who were killed Tuesday Feb. 22, 2011 by Somali pirates after being taken hostage Friday several hundred miles south of Oman. (AP Photo/Del Rey Yacht Club)

A pirate in Somalia who gave his name as Adowe Osman Ali said pirates ferried the reinforcements to hijacked ships in their hands on Wednesday in a bid to deter 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, who referred to the pirates as soldiers.

"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."

Criminal gangs now dominate the piracy trade, and have begun systematically torturing hostages, including tying them upside down and dragging them in the sea, locking them in freezers and beating them. Pirates have also used the hostages as human shields.

"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 fisherman who just decided to have a go at piracy," 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.

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 that there is still a key piece of missing information after the deaths of the four Americans on Tuesday, and it's not clear if the deaths will require a wholesale change in the way the shipping industry and the militaries patrolling the Gulf of Aden and Indian Ocean operate.

The U.S. military said a pirate fired a rocket-propelled grenade at a Navy warship, and that gunfire was heard on board the Quest. Why that violence broke out - whether because of an internal pirate fight or an attempted escape by the hostages - is not publicly known.

"For us it's too soon. 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 on board the Quest 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 don't care too much about our lives when foreign ships are plundering our country's waters," he said. "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."