It has been reported that Mohammadi was working on an international energy project with other Mid-East scientists, including Israelis. Mohammadi in his travels, had informal contact with Israeli scientists. So why would Israel, or the US, want him dead? On the other hand, talking to Israelis is reason enough for the Iranian government to want Mohammadi dead. Mohammadi signing a letter critical of the government is another.
Zimbabwean charged over killing of Cecil the lion (Reuters)
from the article: ANALYSIS / Iran scientist likely killed by opponents of nuclear program