I think there are two variables here:

A: the number of people radicalized into anti-American terrorists

B: the opportunity that As have to cause damage

I suspect A went up in the late nineties and continues to be high-ish today. B went up with OBL and the Saudis funding the game, but has gone down post-9/11 for a variety of reasons, including decreased Saudi bankrolls, increased airport and aircraft security, tighter restrictions on student visas, and a weakened Al Qaeda.