PHNOM PEL, CAMBODIA – More than 100 burial jars and a dozen coffins arranged on a ledge in remote Cambodian jungle have for centuries held the bones — and secrets — of a mysterious people who lived during the Angkor era.

Why the bones were placed in jars on a cliff some 100 meters high in the Cardamom Mountains — or indeed whose remains they are — has long puzzled experts.

For seven years Nancy Beavan, an archaeologist who specializes in carbon dating, has been looking for an answer, painstakingly piecing together clues left by the enigmatic people at 10 sites dotted across the area in southwestern Cambodia....