10^-9
One in a billion? I hope you are right.
Then what accounts for the concerns in the GAO report? I know I'm oversimplipifying, but wouldn't you first need to find the very few guys who could do it, and from them find the tiny portion who would do it?
Are the GAO, the FAA, and the FBI responding to a non-existent threat? If they are, it wouldn't be the first time...