Last night a close friend of Mr Khashoggi revealed that he was about to obtain "documentary evidence" proving clams that Saudi Arabia had used chemical weapons in its proxy war in Yemen.

"I met him a week before his death. He was unhappy and he was worried," said the middle eastern academic, who did not wish to be named.

"When I asked him why he was worried, he didn't really want to reply, but eventually he told me he was getting proof that Saudi Arabia had used chemical weapons. He said he hoped he be getting documentary evidence.

"All I can tell you is that the next thing I heard, he was missing."

While there have been recent unsubstantiated claims in Iran that Saudi Arabia has been supplying ingredients that can be used to produce the nerve agent Sarin in Yemen, it is more likely that Mr Khashoggi was referring to phospherous.