How could you know their motivations either way? It's not as black and white as that. There were probably multiple causes (Western foriegn policy, Israel, poverty, sexual repression etc) but to deny Islam could have had any influence on them, when they admit themselves that their actions are in the name of Allah, is fatuous.
@RedSparrows Those who bombed the twin towers used Islam as a crutch for their own anger and hatreds. Just like (for example) Westboro Baptist Church use Christianity as a proxy for their own hatred, and just how America uses 'democracy' as it's motives despite in practice overthrowing a great many democracies for dictators when they feel like it.