... vividly described with fascinating twists and turns. Hagedorn has clearly devoured a mountain of research, so it’s disappointing when, writing about the discovery of nuclear fission, the author credits 'two German physicists in Berlin in late 1938.' In fact, although the experiments were run by the German scientists, the revolutionary interpretation that the atom was being split was made by an Austrian Jewish woman who had fled to Sweden. Lise Meitner should have been correctly credited, especially given that one of Hagedorn’s most important sources was a scientist colleague of Koval’s, Arnold Kramish. Kramish wrote a book about another atomic spy, this one for the Allies, who used Meitner to get information about what her colleagues were doing in Germany since they stayed in touch through letters. This lapse makes the reader question the accuracy of other assertions throughout the book. It’s a good adventure story, well told, but how reliable is the research that underpins it when such an error is included? ... What is certain is that Koval successfully passed on information that shortened the time the Soviets needed to make their own atomic bomb. Equally clear is how anti-Semitism ran through both the United States and its rival, and how that hatred affected scientists on both sides. The story of how Koval’s involvement was ultimately revealed is compelling, but the even more gripping narrative is how and why he became a spy in the first place. Hagedorn navigates her way through the many layers of deceptions, telling a story worthy of John Le Carre.

... compelling ... Throughout this narrative, readers should expect many unanswered questions since key information on Koval remains inaccessible. Nevertheless, Hagedorn’s well-researched account employs a host of primary and secondary sources to convincingly connect the dots between Koval, the Soviet spy network, and the creation of the atomic bomb.

Hagedorn effectively tells how Koval returned to the Soviet Union in 1948 to live quietly and eventually receive posthumous recognition. This expertly researched, psychologically thrilling history makes good use of available primary sources, such as FBI reports, letters, and interviews, as well as secondary literature ... Engaging narrative nonfiction that will thrill readers who are drawn to works by Ben Macintyre and Kate Moore.

Hagedorn draws from decades of scholarship, thorough archival research, and deftly FOIA’d F.B.I. reports to reconstruct the intricate network of Russian cells in midcentury America, a world of handlers and honey traps and tradecraft worthy of a prequel to The Americans. A veteran of The Wall Street Journal, Hagedorn is an impressive reporter, sparing no detail from the elements of the story to which she has access. And it’s without question a great story. The problem is that some of the juiciest bits—notably any real sense of Koval’s interiority, or the specific way he communicated intelligence to his handler—didn’t make the historical record. This is the fundamental challenge of writing about espionage: spies work hard not to leave a trace. And Koval was by all accounts an exceptional spook. Hagedorn is often left to speculate, relying on qualifiers such as 'must have' or 'surely.' Such sleights of hand are inevitable in narrative nonfiction—Ben Mcintyre’s Cold War histories are full of them—but in this case one can’t help wondering whether Koval successfully foiled not only the American authorities but also any attempt by posterity to satisfactorily account for his role in history.

... doggedly researched ... Enlivened by its brisk pace and lucid scientific details, this is a rewarding introduction to a noteworthy episode in the history of Soviet espionage.