I would consider it unlikely for the attack to be done in that manner by anyone other than Islamic fanatics. The perpetrators of 9/11 (20 people, and possibly more teams that didn't get to act) were all planning on suicide. That takes a cartain mentality, and I can't think of any other ideology that would generate that many fanatics, ALL eager to die. That doesn't mean that they can't be someone else's patsy's. Suicide bombing was also new then, compared to routine now.
Note that, when I was writing a massive terrorist attack in April of 1980, I chose a method that required either one or no suicide attackers.
I'm watching this thread with interest for ideas for The Masquerade.