The US has a policy of no assassination of civilians. They do assassinate terrorist combatants and their senders since they participate in a "war". If the US did assassinate the Iranian scientist, this would have been a foolish departure from policy, because killing scientists is just too easy, especially scientists in democracies. I hope it isn't true, but if the Iranians are correct, the authorizing US official should pay the price.
Army: Blast in Korean DMZ wounds two South Korean soldiers (Reuters)
from the article: What if the Iranians start killing scientists?