Speigel Online says they have a source who confirms it's the Mossad that's been carrying out the assassinations over the last two years of Iranian nuclear scientists:

There is little doubt in the shadowy world of intelligence agencies that Israel is behind the assassination of Darioush Rezaei. "That was the first serious action taken by the new Mossad chief Tamir Pardo," an Israeli intelligence source told SPIEGEL ONLINE.

On July 23, Rezaei became the latest victim in a mysterious series of attacks over the past 20 months which has seen the virtual decimation of the Islamic republic's elite physicists. The 35-year-old died after being shot in the throat in front of his daughter's kindergarten in east Tehran. The Iranian press has reported that the two alleged perpetrators in the attack escaped on a motorcycle.

So who was the man who was shot in front of his wife and daughter? According to the Associated Press, the International Atomic Energy Agency (IAEA) in Vienna has since confirmed what had already been reported by the Israeli media -- namely that the physics student had worked on the development of high-voltage switching systems, a key component that is crucial to setting off the explosions needed to trigger a nuclear warhead. The physicist had apparently been seen daily at a nuclear research center in northern Tehran.

The fact that Rezaei's death has struck a nerve in Iran is apparent in the official reaction to the killing. Kazem Jalali, the head of the Iranian parliament's national security committee, said the murder of Iranian physicists showed that the United States and Israel are "desperate" in the face of Tehran's nuclear ambitions.

There have been three assassinations over the last 2 years of important nuclear scientists.

The problem I have with assigning responsbility to Mossad is that it is almost as likely that the scientists were killed by Iranians. Factional fights in the Iranian leadership is often deadly business and each of these scientists were involved in important work for Supreme Leader Khamenei. It's been suggested before that there are elements in the Iranian leadership who are unhappy about the ungodly amount of money being spent on the nuke program, and who believe that slowing it down by killing scientists would embarrass Ahamdinejad and his boss Khamenei.

We'll probably never know.