There's a decent chance that Iran/Iranians are who did this, given that putting your president in a helicopter through fog would be, well, not a good idea. You also need knowledge of when and where he's flying for an attack, when and what method for a bomb/sabotage, info that Iranians would be the most likely to have.
Azerbaijan as a country I'd say is a little less likely, as it killed their own diplomats, but I could see an Azerbaijani faction doing this.