It's been four days since Iran's top nuclear scientist, Mohsen Fakhrizadeh, died in a hail of bullets on a highway near Tehran. Iran has plausibly blamed Israel for the killing, but more than that, not much is known credibly or in detail.

This is hardly the first time that an Iranian nuclear scientist has been assassinated in an operation that has a whiff of Mossad about it. But Fakhrizadeh's prominence — he is widely regarded as the father of the Iranian nuclear program — as well as the timing of the killing, just six weeks from the inauguration of a new American president, make it a particularly big deal. Not least because an operation this sensitive would almost certainly have required a US sign-off.