A man suspected of beheading a businessman at a French gas factory in June has been found dead in his prison cell.

A man who beheaded his boss, pinned the head on a fence, and tried to blow up an industrial gas plant will be investigated on terrorism charges, the chief public prosecutor in France said, dismissing the suspect’s claim that his act was not motivated by connections with Islamist militants.

Even as French investigators piece together a clearer picture of events that led on Friday to the beheading of a transport firm manager and attempted destruction of a chemicals plant, the real motivation of the chief suspect remains elusive.