Thanks for making me discover who
Marinus van der Lubbe was.
I say it was mexicans crossing the border!
PROOF:
In one of those special episode zero from 24, you see the terrorist crossing the US-mexican border!
It was the one who almost blew up Los Angeles with an atomic bomb.
(Personally, I think it should have worked. It would have ended this show in a fantastic way.)
edit:
AND YOU FORGOT THE TRAINS!!!
I personally don't believe in this theory, but it should not be excluded in a full study of this attack.