Who is assassinating Iranian scientists and engineers?

Darioush Rezaie, 35, a university lecturer, was shot dead by gunmen in eastern Tehran Saturday.  According to Reuters:

Western security agencies were most likely behind the killing of an Iranian scientist in an operation that underlines the myriad complications in the conflict over Iran's nuclear program.

Al Arabiya states (quoting Irans Student News Agency):

“An Iranian nuclear scientist was assassinated in front of his house today ... and his wife was also wounded,”

Al Arabiya adds:

There have been several instances when Iran’s nuclear scientists were targets of assassination or kidnap. 

In November 2010, twin blasts in Tehran killed top nuclear scientist Majid Shahriari and wounded another, Fereydoon Abbasi Davani. Tehran then swiftly blamed the CIA and Mossad for the attacks. 

In January 2010, Masoud Ali Mohammadi, another Iranian nuclear scientist involved with the SESAME- light for Experimental Sciences and Application in the Middle East - project, was killed in a bomb attack which Tehran blamed on “mercenaries” in the pay of Israel and the United States. 

In July 2010, Iranian nuclear researcher Shahram Amiri said after returning to the Islamic republic that he had been held in the United States for more than a year after being “kidnapped” at gunpoint by two Farsi-speaking CIA agents in the Saudi city of Medina.

On the other hand, Science magazine reported in its January 22 edition of 2001 that Masoud Ali Mohammadi was not a nuclear scientist (although he was a physicist), and that

Alimohammadi was one of 240 Tehran professors who had declared their support for Ahmadinejad's main opponent, Mir-Hossein Mousavi.

It goes on to suggest that students at his university attribute his assassination to officials of the Islamic Republic. There are also reports, including those citing the Government of Iran that Rezaie was not a nuclear scientist.

I would note that the Welfare of Scientists network is working on a number of cases involving Iranian intellectuals and scientists.