The answer lies deep in the heart of Africa. It was extracted from the Shinkolobwe mine in then Belgian Congo, owned by Union Minière, part of the mother country’s biggest and wealthiest company, Société Générale de Belgique. Without access to that mine, the atom bomb might never have been built by American scientists during the second world war.

Susan Williams’ previous book, Who Killed Hammarskjöld?, prompted the United Nations to set up a new inquiry into the death – in a plane crash in Northern Rhodesia – of its former secretary general . Her new, meticulously researched book has shades of Graham Greene, a hint of Conrad , even echoes of Indiana Jones.

Using newly opened archives and personal interviews, she describes how the American Office of Strategic Services (OSS) – forerunner of the CIA – recruited a motley band to ensure the uranium reached the US and did not fall into the hands of Nazi Germany. The group of hand-picked officers included two ornithologists – good training, perhaps, for a spy. They posed as rubber prospectors or investigators of diamond smugglers.

The word “uranium” never passed their lips: instead, even in coded telegrams, they referred to “raw material”, “diamonds”, or, simply, “gems”.They were not told why the US was so desperate to secure a regular supply of the high-grade uranium ore. Not even Wilbur Owings Hogue, the OSS station chief in Belgian Congo, described by Williams as “a very clever civil engineer with a self-deprecating sense of humour”, was told.

Their task was made more difficult and dangerous by pro-German sentiments among some of the Belgian industrialists and colonists in the Congo. Union Minière had already sold significant amounts of uranium ore to Germany. However, the US had a key ally in Edgar Sengier , head of Union Minière’s New York-based operations, who after the war was awarded the medal for merit, which was then the US government’s highest civilian award.

In Congo, many businessmen were waiting to see who looked like winning the war before committing themselves to the Allies. Senior Belgian officials were themselves involved in diamond smuggling, sometimes in Red Cross parcels, for their personal benefit. There were always doubts about their loyalties; few could be trusted. Using diamond smuggling as cover for their true role remained a potentially fatal strategy for OSS officers. Hogue’s life was threatened more than once, as a result of leaks from a Belgian journalist he recruited as an informer.

Williams contrasts the luxurious living of Congo’s white inhabitants with the brutal treatment of the black population. Soon after the end of the war, in November 1945, strikers protesting at terrible living and working conditions in Léopoldville (now Kinshasa, capital of Democratic Republic of the Congo) and the port of Matadi (through which much of the uranium was shipped) were killed along with women and children. Workers at the Shinkolobwe mine were not protected at all from radiation.

“The moral authority of the struggle against fascism was not applied to the inequalities and injustice in the Congo,” Williams writes.

If I have a quibble about Spies in the Congo it is that, on occasion, dense – albeit rich – detail slows down the pace of what is truly a thriller, in which Williams paints clear and sympathetic pictures of characters thrust into a totally unfamiliar territory.

She opens her book with a letter Albert Einstein wrote to then US president Theodore Roosevelt on August 2, 1939, alerting him to the possibility of constructing a powerful bomb turning uranium into a new and important source of energy. “The most important source of uranium is Belgian Congo,” Einstein wrote.

There is another terrible legacy, aside from the bombs dropped on Japan, at the source of that uranium. Radioactive waste at Shinkolobwe has led to birth defects and cancers. After the collapse of a mine shaft, the UN asked the International Atomic Energy Agency to investigate conditions there . Its study found that 6,000 small-scale miners had been digging for coltan and cobalt: everyone working there was at risk of developing health problems because of high levels of radiation.

“Shinkolobwe has never been commemorated,” the Congolese journalist Oliver Tshinyoka noted last year - the 70th anniversary of the atom bomb attacks on Japan. This book might help to change that.

Richard Norton-Taylor