Who killed Iranian nuke physicist Masoud Alimohammadi?

Remember Masoud Alimohammadi, the Iranian nuclear physics professor who was killed by an exploding motorcycle last month? At first, it was assumed that because he was a supporter of the Green revolution and Mir Hossein Mousavi, he was killed by the Ahmadinejad regime. Now, it turns out that he was an important cog in Iran's nuclear program and that someone else may have wanted him dead.