I don’t absolutely know if it’s true, but there were several stories back then about Chinese modifications of the shoulder held missiles of that time, which could have reached the required altitude.
So I don’t think it can absolutely be said that a terrorist missile can be ruled out on that basis.
Hello? There were "stories" about Chinese modifications? Have any of these missiles been seen before of since? And even if you had a missile that might be able to hit something at 20,000 feet, why not position yourself to shoot at the plane when it is at 8000 feet?
The Navy had restricted flying near the place where TWA 800 went down. The Navy first denied they had assets in the area. Then after they did admit that there were assets in the area, they didn't answer many questions about what those assets were doing there. In fact, few such questions were asked.
Besides the Navy being in the area, doing something or other, we have a Coast Guard. Certainly a missile fired from the water close to our shore, should arouse their interest. So far as I know it did not.
Doesn't any of this bother you?