Prosecutor General Saeed Mirza said in a statement that Shaker al-Absi did not die when the Lebanese army stormed the Nahr el-Bared refugee camp in northern Lebanon, where the al-Qaeda inspired militant group had barricaded itself.
He said that DNA tests on a body identified as Absi's by his wife conclusively proved that it was not the militant leader.
According to Mirza, a captured Fatah al-Islam fighter told authorities that al-Absi escaped the encirclement on the night of September 1, and that he was presumed to be alive and in hiding.
Dozens of Fatah combatants were killed September 2 when they attempted a mass breakout in the face of an all-out Lebanese army assault.
At the time, Lebanese Prime Minister Fouad Siniora called the capture of the refugee camp Lebanon's "biggest victory over terrorists."
Ending a siege that lasted three months, the army recaptured the camp after killing 37 militants making a final, desperate attempt to break out in the pre-dawn darkness.
It was reported that the group's leader, Shaker al-Absi, was among the dead, and his supposed remains were identified by his wife at a hospital morgue in Tripoli.
More than 220 people, including 158 Lebanese troops, were killed during the standoff, which began May 20 near the sprawling camp.
In a televised speech to the nation following the assault, Siniora said: "You (the army) have achieved at Nahr el-Bared the country's biggest victory over terrorists."
He pledged that the Lebanese government would rebuild Nahr el-Bared, but said that the camp would be placed under the authority of the state and "only the Lebanese state."
