The latest attacks in Bulgaria are supposed to be in retaliation [1] for past assassinations [2] [3]
[1] http://www.haaretz.com/news/diplomacy-defense/u-s-officials-hezbollah-attacked-israelis-in-bulgaria-to-retaliate-for-killing-of-iran-nuclear-scientists-1.452410
[2] http://rockcenter.msnbc.msn.com/_news/2012/02/08/10354553-israel-teams-with-terror-group-to-kill-irans-nuclear-scientists-us-officials-tell-nbc-news
[3] http://www.telegraph.co.uk/news/worldnews/middleeast/iran/9266889/Iran-hangs-Israeli-agent-for-killing-nuclear-scientist.html