|If it quacks like a duck...|
In the case of homegrown terrorists, particularly that very large fraction who do not survive their attack to be questioned, motivation is a difficult thread to unravel. Someone raised a Muslim, however, who at some point claims the attack is driven by Islamic fundamentalist or Muslim nationalist ideologies, must be taken at their word that their religious and ethnic indoctrination had some influence on their actions. In the case of the Orlando terrorist, for example, the attack was premised on his hatred for gay people, but it seems reasonable that the root of that hatred - whether for gay culture or self-loathing at his own sexual impulses - was the Islamic fundamentalist indoctrination via his father.
The fact is that, however you feel about the surveillance state and Homeland Security tactics, the US is a very difficult target for overseas terrorist organizations to get to. To move people, funds and weapons into the US, organize and coordinate them for a large scale attack is almost impossible to do without detection and interdiction. The only way to carry out an effective terror attack in the US today without an inordinate amount of luck is if the attacker(s) have NO communications with any known terrorist organization.
The lesson seems to be that people with a predisposition to mass killing will find some justification for doing it. The existence of Islamic terrorist ideology provides people who were raised in that faith (or converted out of some sympathetic feelings for the dogma) with a ready-made formula for both motivation and justification. And with the key factor determining whether someone carries out such an attack being a willingness/desire to die in the process, a motivation rooted in religious mythology that includes eternal life in paradise can be an important enabling factor. But it seems substantially less is important whether the attacker(s) were an active part of the terrorist organization or merely aware of their worldview and doctrine and sympathetic to their cause. Indeed, it is becoming increasingly obvious that the successful mass casualty attacks will have that specific factor in common - zero communication with any extant terrorist organization before the attack. That kind of operationally secure mission planning is the best way to avoid coming to the attention of law enforcement or Counter Terrorism personnel.