Foreign states have the resources to build their own field tests.
But time will tell. In fact it already has. We just need to wait for the investigation to complete.
My own money would be on a terrorist organization. A bit less certain that it would be domestic, but domestic is still more probable than foreign in my mind. The past few years, and the past year in particular, has shown me that our domestic terrorists on the left and the right are bat$#!t crazy.
But as I said, we'll see.