I agree there are a lot of incompetent want to bes out there, especially the self actualized the ones who are limited to training via the internet. However, I disagree that without State support we don't have to worry about a serious terrorist attack. Of course depending on your view of serious. I would consider Tim McVeigh's (sp?) attack on the Federal Building in Oklahoma a serious attack without a State sponsor. There are plenty of opportunities for ex-military members/police from numerous nations to train indepedent actors (with no state sponsors) to be fairly competent with small arms and to develop home made explosives. The Mumbai attack was state sponsored, but it definitely doesn't have to be. I think of two incidents off hand where one or two attackers without State sponsors caused a lot of mayhem. The bank robbers in L.A. a few years back who wore head to toe body armor and held off a number of police officers for an extended period of time. If it was their intent to kill civilians and a lot of them, they easily could have. The kid at the college/university in W. VA who managed to kill several students before the police responded. Some consider MAJ Hansan's attack in TX a serious attack. A couple of other attacks come to mind overseas. First the chemical in Japan by Aum Shinrikyo, and the right wing Italians who denotated a large bomb in a train station in Bologna that killed over 80 people.

If you're asking if a non-state sponsored group can conduct a 9/11 level attack, I think the answer is a definite yes, but those attacks will outliers not the norm, because most people who gravitate to terrorism are relatively incompetent to begin with, but there are exceptions and for those exceptions there are opportunities for mass mayhem.