Best remembered for the mysterious circumstances of his death, this Welsh-born mathematician was a dual Government Communications Headquarters (GCHQ) and British Secret Intelligence Service agent.
After studying at Bangor University in his late teens, he earned a PhD in mathematics from the University of Manchester.
His naked body was found stuffed in a large bag in a British secret service safe house in the Pimlico neighborhood of London, England.
Born and raised in Anglesey, Wales, he grew up speaking Welsh as his first language and English as his second.
On the request of the U.S. State Department, British Foreign Secretary William Hague signed a document after Williams' death that barred the general public from many of the details of Williams' professional life as an intelligence officer.