The first clue surfaced in September 1998, when fishermen off this Mediterranean port city dragged up a silver bracelet with their nets. It bore the names of Saint-Exupéry and his New York publisher. Further searches by divers turned up the badly damaged remains of his plane, though the body of the pilot was never found.

Image Saint-Exupéry was one of numerous French pilots who assisted the Allied war effort. Credit... Agence France-Presse

“I had just seen ‘Titanic’ and after a few glasses of pastis I reflected, ‘We’ll make a movie, and the dollars will rain,’ ” said Jean-Claude Bianco, 63, on whose boat the bracelet was discovered.

The film, was never made but news of the bracelet prompted Luc Vanrell, 48, a diving coach and marine archaeologist, to inspect more closely some marine wreckage he had noticed years before, buried in sand in 170 feet of water near the remains of Saint-Exupéry’s plane. An engine block serial number and a Skoda symbol, for the Czech company that was an unwilling German supplier, proved it to be a Daimler-Benz V-12 aircraft engine.

In 2005, after enduring numerous bureaucratic delays, Mr. Vanrell and another diver, Lino von Gartzen, lifted the motor and shipped it to Munich for study by German experts. It turned out to be part of a series produced in early 1941  the oldest sparkplug was from March 1941. It had been modified in 1943 with the addition of a Bosch fuel injection pump.

The researchers deduced it had powered a Messerschmitt fighter plane, part of a training unit stationed in southern France from 1942 to 1944. It had been flown by Prince Alexis von Bentheim und Steinfurt, a 22-year-old who was shot down by American planes in late 1943, on his first and last solo flight. The tale might have ended there, with the death of the prince and of the Little Prince’s author. Yet Mr. von Gartzen was not content. Consulting archives and with the help of the staff of the Jägerblatt, a magazine for Luftwaffe veterans, he tracked down veterans who had flown in Prince von Bentheim’s unit, the Jagdgruppe 200. He contacted hundreds of former pilots, most now in their 80s; hundreds more had already died.