DNA tests on a body earlier identified as the leader of Islamist militants in a refugee camp stormed by the Lebanese army last week have proved negative.
Shaker al-Abssi set up Fatah al-Islam in northern Lebanon last year
Prosecutor-general Saeed Mirza said the tests countered the identification of Fatah al-Islam leader Sheikh Shaker al-Abssi's body by his wife last week.
Mr Mirza said he now believed Abssi fled the Nahr al-Bared refugee camp a day before the army stormed it.
More than 400 people died during the 105-day siege, half of them soldiers.
The violence, which also caused more than 30,000 Palestinian refugees to flee the camp, was Lebanon's worst internal strife since the 1975-1991 civil war.
Earlier, Lebanon's Prime Minister, Fouad Siniora, opened a conference in Beirut aimed at raising funds to rebuild Nahr al-Bared.
Mr Siniora said $382 million (£188 million) was needed to allow residents to return to their homes. In the meantime, many have been living in cramped conditions in another refugee camp near Tripoli.
In a statement, Mr Mirza said the DNA of the body being stored at the Tripoli Governmental Hospital did not match that of Abssi's children or brother.
Split from Palestinian group Fatah al-Intifada in late 2006
In May, had 150-200 armed men in Nahr al-Bared camp
Denies al-Qaeda links but says it endorses its ideas
Has links with Syrian intelligence, Lebanon says
He then revealed that a captured Yemeni militant had told police that he had escaped Nahr al-Bared on 1 September with Abssi and several others.
"Shaker al-Abssi was in good condition and was wearing a suicide belt and carrying a Kalashnikov rifle, magazines and hand grenades," the statement quoted the Yemeni as saying.
A well-known Palestinian militant, Abssi was sentenced to death in absentia in Jordan for killing a US diplomat, Laurence Foley, in Amman in 2002.
He was later jailed in Syria before he set up Fatah al-Islam in Nahr al-Bared last year.