Contra Frum, Eli Lake reports that Israel is the most likely suspect:
Patrick Clawson, the director of research at the Washington Institute for Near Policy, said the signs point to Israel. "This sophisticated technique is uncharacteristic of the Iranian armed opposition and the Iranian government, it is characteristic of the Mossad," he said.
"I am unaware of episodes when Americans and Europeans have done this kind of assassination. Of course, the Americans are involved in assassinations using predators, but not this kind of operation with agents on the ground, the natural suspect is the Mossad."
A former Mossad officer now living in Canada who goes by the pseudonym Michael Ross said the attacks bore the hallmarks of an Israeli operation. "This tactic is not a new one for the Mossad, and worked very effectively against Egypt’s rocket program in the 1960s. During that period, the scientists involved in that project were assassinated and the program suffered immensely."