Mohsen Fakhrizadeh: key figure in Iran's nuclear efforts who avoided limelight

Analysis: engineer was identified by UN report as central figure in work to develop atomic bombs

• Iran's top nuclear scientist shot dead near Tehran

Outside of the circles who closely watch Iran's nuclear programme in Israel, the US and at the International Atomic Energy Agency, Mohsen Fakhrizadeh, who has been shot dead in Tehran, was not a household name.

But for two decades the nuclear engineer - once a familiar sight to his students at Tehran's Imam Hossein University, where he lectured in physics - was of interest to nuclear proliferation experts owing to the suspicion that he was a key figure in Iran's nuclear efforts.