Seems to be the drones used have quite a range to have been launched from Yemen.
Everybody could do it. Including Saudis themselves. There are too many parties to benefit. My guess you need to wait for the official line from MSM and suppose the opposite.
Seems the answer is to have security patrolling the skies around major assets, not just refineries, and blast the drones before they get close enough to drop their load. I hope this is at least being considered for major US assets as well.