In particular, Union of Concerned Scientists researchers. But they're not really researchers, because they'll never report results that don't support UCS political positions.
Also, "analysts" from Middlebury Institute of International Studies — apparently another nonproliferation shop.[1]
Also, "analysts" from Middlebury Institute of International Studies — apparently another nonproliferation shop.[1]
And finally, anonymous "U.S. officials".
[1] http://www.miis.edu/about/newsroom/experts/jlewis