Beirut, Lebanon ? A Palestinian guerrilla group accused the United States on Wednesday of assassinating its leader Abul Abbas, and a U.S. Pentagon official said the United States believed he died of a heart attack.

Abbas, 56, died Monday in U.S. detention in Baghdad. He was known for leading the 1985 hijacking of the Achille Lauro passenger ship in which a wheelchair-bound Jewish American tourist, Leon Klinghoffer, was killed and thrown overboard.

The U.S. deputy chief of operations in Iraq, Maj. Gen. Mark Kimmitt, said Abbas almost certain died of natural causes and an autopsy would confirm that. The U.S. Pentagon official said the autopsy had not been performed but that officials believed the cause was a heart attack.

Abbas’ deputy in the Palestinian Liberation Front, Omar Shibli, said Abbas had never complained of ill health in letters he had sent in recent months.

“Abul Abbas’ detention was illegal and his condition in the jail was very bad,” Shibli said. “The Americans treated him in a way that led to his death in his prison cell.”

The Palestine Liberation Front issued a statement in Beirut saying the United States had wrongly arrested Abbas and assassinated him.

The statement said his “assassination” after being arrested without justification “confirms beyond any doubt their absolute hostility to our people and exposes their designs which conform with the Zionist entity.”

Abbas was captured in south Baghdad on April 15, days after U.S.-led forces overthrew the regime of Saddam Hussein. He spent his last 11 months in American custody.

Abbas’ widow, Reem Nimer, told Associated Press Television News on Wednesday she wanted the Americans to explain how he died.

“They know how Abul Abbas died a martyr. Was he deprived of a medicine which led to a deterioration of his health? Or did he suffer a sudden stroke? Or was he tortured? We have the right to ask all these questions and they must answer them,” Nimer said, with tears in her eyes.

Nimer, in her late 40s, lives with her 20-year-old son, Ali, in Beirut. She rejected labeling her husband as a terrorist, saying he fought for his homeland, Palestine.

“The Zionist and American propaganda accused him of being a terrorist while he was a patriotic man committed to his national cause,” Nimer said.

Nimer has met an official of the International Committee of the Red Cross to discuss retrieving Abbas’ body from Iraq.

She said she wanted her husband to be buried in the Palestinian territories, but if Israel rejected that, “we will look for any Arab country.”