“Was I — quiet, harmless Martha Cnockaert — really about to do this terrible thing?” (The book used the English spelling of her first name.)

McKenna said she was caught after she had aided in the explosion of a weapons stockpile. There she lost a wristwatch engraved with her initials, and the Germans, suspecting its owner to be the culprit behind the explosions, put up a notice saying they had found it. She claimed it, falling into their trap.

The Germans then searched her home, found hidden coded messages and arrested her on espionage charges in November 1916. Though the penalty for spying was usually execution, McKenna was imprisoned instead because, she said, of her work as a nurse and the Iron Cross she had received.

She later received honors from France and Belgium and was hailed in dispatches by Field Marshal Douglas Haig of Britain, who included her on a list of Belgians who had provided “distinguished and gallant services.”

After the war she married John McKenna, a British officer, and they moved to England. Historians believe her husband was probably the ghostwriter for “I Was a Spy!” though publicly the couple claimed that Marthe McKenna had written it. The book initially sold about 200,000 copies and received rave reviews.

“From first page to last it is a thrilling, breathtaking book,” The New York Times wrote. The Sunday Dispatch in England printed excerpts, describing the book as the “greatest of all war stories” and comparing McKenna to Joan of Arc.

But how much of it is true remains a question.

Coghe, the historian, said many parts of the book were fictionalized and that some events combined McKenna’s experiences with those of her aunt.