Palestinian militant Abu Abbas, mastermind of the 1985 Achille Lauro cruise ship hijacking, has died in US custody, US officials have confirmed.
Abbas was captured in Iraq in April 2003
Abbas, leader of the Palestinian Liberation Front, apparently died of natural causes, officials said.
He was convicted in absentia by an Italian court for the attack, in which wheelchair-bound American tourist Leon Klinghoffer was killed.
Abbas was captured in Baghdad last year by US special forces.
Pentagon officials said that Abbas, who was in his mid-50s, had died on Monday, most likely of a heart attack.
They said an autopsy would be performed to determine the cause of death.
Abbas described the killing of Klinghoffer as a mistake
The BBC's Nick Childs at the Pentagon says that as the leader of the Palestine Liberation Front, Abbas could have had information that could have been of interest to the United States in its so-called global war on terrorism.
Abbas was one of several PLF members who carried out the 1985 attack on the Achille Lauro, in an attempt to secure the release of 50 Palestinian prisoners being held in Israel.
The ship was seized while sailing between the Egyptian cities of Alexandria and Port Said.
During the two-day stand-off the gunmen killed Klinghoffer, an elderly US Jew confined to a wheelchair, and threw his body overboard.
Klinghoffer's family said on Tuesday that Abbas' death had robbed them of justice for the killing.
"The one consolation for us is that Abu Abbas died in captivity, not as a free man," a statement from the family said, as quoted by French news agency AFP.
After the stand-off, Egypt gave free passage to the hijackers in exchange for the rest of the hostages - many of them Americans.
But the plane carrying the hijackers to Tunisia was intercepted by US Navy jets and forced to land in Italy.
Abbas' co-conspirators were sentenced to long prison terms, but he himself was freed by the Italian authorities, who said they had insufficient evidence to detain him.
However, he was later convicted in absentia of masterminding the hijacking and received five life sentences.
Abbas reportedly spent around 17 years in Iraq before his capture there in 2003.
In 1996, he described the killing of Klinghoffer as a mistake.