The chemistry expert, Mostafa Ahmadi Roshan wqs killed by a magnetic bomb that was attached to his car by a man on a bicycle. Clear evidence of a covert operation and not some random act of violence. Why? Because Roshan was working as the director of the Natanz uranium enrichment facility in central Iran.
The Iranian government was quick to blame Zionists and the British government for the attack though both countries denied involvement. Of course that is the nature of "covert" operations. Still signs point to Israel and specifically because of a statement made by Israeli Lt. Gen. Benny Gantz who told a parliamentary panel that 2012 would be a "critical year" for Iran — in part because of "things that happen to it unnaturally." More of the story in USA today.
Israel has show a willingness to carry out assassinations in the past and has a well organized secret operations arm of the Mossad. Iran accused Israel of the cyber attack in 2010 using the Stuxnet virus. It seems like a likely scenario, even though everyone denies involvement.
In the Middle-East, Israel has always had to be scrappy and defensive and for good reason, but today they have a lot of power and very well organized intelligence. It remains to be seen if they can slow the Iran nuclear program or whether they will only succeed in strengthening Iran's resolve to have the bomb.
I am reminded that there already is a nuclear power in the Middle-East, and that is Israel.