According to a Washington Post article, this Iranian scientist was working on the SESAME project. This project was a joint collaboration with Israeli, European, and Iranian scientists. Israeli scientist had spoken to and worked with Dr. Massoud. I believe that the Iranian scientist was killed by the Iranian Basiji.
Trump: Middle East would be more stable if Saddam, Gaddafi still in power (Reuters)
from the article: ANALYSIS / Iran scientist likely killed by opponents of nuclear program