If I were an enemy of the US, one of the tools in my bag would be to have operatives well trained as aviation mechanics and join the military or civilian airline industry. There are many ways to subtly sabotage aircraft. The operative may do nothing for years before getting the word to perform their nefarious deed. Seldom will they be detected for whatever sabotage done, the evidence is destroyed in the crash.
You mean like the ones who now are allowed to wear turbans?