Recall the 9/11 bombing and the ongoing hundreds of Islamic jihad attacks around the world and the immediate announcements of credits from the jihad warriors to the world.
Unless the jihad is changing it's tactics of announcing glory to Allah for their strategic strikes, the jihad warriors are becoming even more clandestine (and deadly).
The culprit(s) may not be known for some time.
It in fact was quite some time before the saboteurs responsible for the 1916 Black Tom explosion were identified.
Personally I think you have hit the nail on the head. I think they know that at least in this country, if they started claiming responsibility for Incidents in the US, that would be the straw that broke the camels back for most Americans, and if you even looked like an Arab you would have to leave this country or face the almost certainty that your days on this planet were numbered.
The problem is there apparently are very few in the US, unless they're the world's laziest and unmotivated terrorists.
You've got several elements in the US:
1) A small (probably fewer than 300 actual agents worldwide and many have been arrested, with fewer than 100 in the US at their absolute peak) but extremely dangerous, fanatical and organized group, Al Queda, which finally came up with a brillant and simple plan to hijack aircraft and use them as bombs. This group seems to have been heavily disrupted.
2) A tiny handful of nutty Muslim hangers-on not affiliated with the AQ organization perpetrating small-time stuff (LAX shooter, and the DC snupers)
3) A much larger network of basically fundraisers for violent Palestinian anti-Israel terrorism, which has never mounted an attack on US soil and never has really seemed to have the inclination to (The South Florida professor, etc.) This is pretty similar to the thousands of Irish-American idiots who donated vast sums of money to, and in some cases purchased and smuggled large amounts of weapons for, the Irish Republican Army in the 70s and 80s.
4) May or may not be very small networks of state-supported terrorists from Iraq. These were hilariously incompetent in a Keystone Kops/Three Stooges way in attempted attacks during the 91 Gulf War.
Collectively, it's not many people, not nearly as many as a lot of people on FR would have you believe. And you have millions of Muslims in the US. Given luck and the right plan, some of them obviously can be extremely dangerous.
But the overall lack of attacks drives a lot of the people who are desperately looking for more terrorism in the US nuts...hence they try to turn every breaking incident into a terror attack and get upset at anyone that doesn't agree.
This works the same way as Al Sharpton and Jesse Jackson going to bed every night desperately hoping they'll wake up and find some dumbass white-trash redneck racists dragged a black guy to death behind their pickup truck overnight. It helps their political agenda and confirms their prejudices. And people LIKE having their prejudices confirmed.
Now, as we know, there ARE some dumbass white-trash redneck racists inclined to drag black men to their deaths. But there are VERY, VERY, VERY, VERY, VERY few of these guys...not nearly enough for Jesse and Sharpton.
In the same vein, there aren't enough fanatical Muslim terrorists in the US for a lot of people....the people that are eagerly sitting around with their Jpegs of atomic bombs going off so their can make their millionth idiotic "NOOKE MECCA" post or prattle on about World War III between Christianity and Islam.