It would not surprise me at all if the terrorists had accomplices working at the airports or airlines involved. However, I think that the flights would have been chosen a long time ago. Their main considerations were probably to choose flights operated by 75-767s, as these were the types on which their pilots were trained, to choose early flights which had the greatest chance of departing on time due to the fact that the aircraft had remained overnight,so that they could strike in unison, and to choose long-duration flights so the aircraft would have a lot of fuel. Boston was probably convenient because New York lies on the flightpath between there and Cuba. Interestingly, I have read about a study that has been done showing that aircraft which end up crashing have a statistically significant higher number of "no-shows" than aircraft which don't; although I have never personally seen it.
