Abul Abbas, the Palestinian mastermind of the 1985 hijacking of the Achille Lauro passenger ship in which an American tourist was killed, has died in US custody in Iraq.

Abbas’ small Palestine Liberation Front commandeered the Italian cruise ship, demanded the release of 50 Palestinian prisoners in Israeli jails and threw an elderly wheelchair-bound Jewish American tourist, Leon Klinghoffer, overboard after shooting him.

US Defence Department spokesman Bryan Whitman said that Abbas, 56, died on Monday, “apparently … of natural causes”. He said there would be an autopsy.

Whitman declined to answer further questions, including whether Abbas was still being interrogated before his death.

Abbas, whose given name was Mohammed Abbas, was captured in southern Baghdad by US forces in a raid in April and lived the last 11 months of his life in American custody.

Abbas’s death was initially announced by officials in Palestinian leader Yasser Arafat’s office in Ramallah.

No cause of death was given either by the Palestinians or the Americans.

When Abbas was captured last spring, the Palestinian Authority demanded his release, saying the United States had pledged not to prosecute him as part of a blanket agreement not to press charges against Palestinians who acted against Israel before interim peace accords were signed in the 1990s.

The United States also endorsed a 1995 interim peace deal which grants Palestine Liberation Organisation members immunity for violent acts committed before September 1993, when the two sides signed a mutual recognition agreement.

Abbas had been a marginal figure in the PLO lately. He was a member of the PLO’s executive committee, but left in 1991. His tiny PLF faction has very few followers in the West Bank and Gaza Strip. According to Israel’s Shin Bet security service, the PLF had sent some members to Iraq for military training.

In April 1996, Abbas visited Gaza for the first time, as part of the amnesty offered by Israel. While in Gaza, he apologised for the killing of Klinghoffer.

In 1998, he returned to attend a session of the Palestine National Council, the Palestinians’ parliament-in-exile, for a crucial vote on abrogating chapters of the PLO founding charter calling for Israel’s destruction. In the end, Abbas did not participate in the vote.

At that time, Israeli attorney general Elyakim Rubinstein said Abbas did not pose a threat to Israeli security, and that it would be unreasonable to prosecute him for acts committed before 1993.

US commandos caught Abbas during a raid on the southern outskirts of Baghdad.

Abbas had been convicted in absentia in an Italian court for the 1985 hijacking and sentenced to life in prison in 1986, but never served any time. His arrest came 18 years after seizing the Achille Lauro off Port Said, Egypt.

After Klinghoffer was killed, the other passengers were released after a two-day ordeal and the commandos surrendered to Egyptian authorities, who put them on a flight to PLO headquarters in Tunisia.

US Navy fighters forced the flight down in Sicily. The Italians, to the Americans’ dismay, allowed Abbas to flee to Yugoslavia before a US warrant for piracy and hostage-taking could be served.

Abbas disappeared, and international manhunts and a price on his head failed to flush him out. He next turned up in Gaza after granted amnesty by the Israelis.

While out of the limelight for the past decade, Abbas is believed to have continued plying the terror trade from Iraq until his April capture.

Israeli intelligence officials say the PLF faction under Abbas was a conduit for Saddam Hussein’s payments to the families of Palestinian suicide bombers.

Israel reported earlier this year that it captured several Palestinians who trained at a PLF camp in Iraq and were told by Abbas to attack an Israeli airport and other targets.

Abbas was born in 1948 in the Yarmouk Palestinian refugee camp in Syria after his family fled from their home in Tira, near Haifa, when the state of Israel was created.

He attended Damascus University and graduated with a degree in Arab literature. He also became involved in student politics and in 1967 joined George Habash’s Marxist Popular Front for the Liberation of Palestine.

He fought as a guerrilla, as often against rival Palestinian factions as the Israelis. But Abbas and others felt that group was focusing too much on political philosophy rather than armed struggle.

In 1976, Abbas took his followers to form the Palestine Liberation Front.