Last week’s deadly assault on a US diplomatic mission in the Libyan city of Benghazi was a "terrorist attack” but probably not a pre-planned operation, a senior US official said.The attack, which left four Americans dead, including US Ambassador Chris Stevens, was carried out on the 11th anniversary of the September 11 attacks and amid protests in the Muslim world against an anti-Islamic film.In Washington, where President Barack Obama’s election rival Mitt Romney has criticized the response to the attack, there has been keen interest in whether the attackers were simply an angry mob or an organized gang.On Wednesday, the director of the US government’s National Counterterrorism Center told lawmakers that, while many questions remain to be answered, he was prepared to describe the killings as "a terrorist attack.”AFP