The latest report from the investigation into the Benghazi attack finds no evidence of Al Qaeda involvement, and no evidence of advance planning.

WASHINGTON — The assault on the U.S. diplomatic mission in Benghazi last month appears to have been an opportunistic attack rather than a long-planned operation, and intelligence agencies have found no evidence that it was ordered by Al Qaeda, according to U.S. officials and witnesses interviewed in Libya.

The circumstances of the Sept. 11 attack have become a matter of heated political debate, with President Obama and Republican nominee Mitt Romney clashing in their debate Tuesday about when Obama termed the assault an act of terrorism. But the emerging picture painted by intelligence officials and witnesses differs from the assertions of both sides.

Republicans have zeroed in on possible Al Qaeda ties to the Sept. 11 attack that killed Ambassador J. Christopher Stevens and three other Americans, and have criticized the Obama administration for not saying early on that it was an act of terrorism. But after five weeks of investigation, U.S. intelligence agencies say they have found no evidence of Al Qaeda participation.

The attack was “carried out following a minimum amount of planning,” said a U.S. intelligence official, who, like others, spoke on condition of anonymity to discuss a matter still under investigation. “The attackers exhibited a high degree of disorganization. Some joined the attack in progress, some did not have weapons and others just seemed interested in looting.”