The construction crews were digging up a patch of land near Northwest 71st Street when they unearthed the unexpected: wrist bones, a human skull, skeletons of two small children. Then came crumbled headstones, nails, buttons and metal coffin handles.

The crew, working on an affordable housing project in the shadow of Interstate 95, had stumbled upon an apparently long-forgotten burial ground -- and a tantalizing puzzle that has stumped Miami's most knowledgeable archaeologists and historians: Who was buried there, and when? How did the graveyard come to be erased from history and memory? And how and where should the remains be re-interred?

So far, answers have proven elusive. An extensive search since the plot came to light in late April has turned up no names, no records, no official documents indicating there was ever a burial ground on the site -- only two commercial maps, from 1925 and 1936, labeling the place a cemetery.

Citing family lore and personal memories, some longtime Miamians recall an informal burial ground for blacks at the site, which sits just east of I-95, sandwiched between 71st Street and the Florida East Coast Railroad tracks.

A preliminary analysis of the bones suggests they may have belonged to black people, but the conclusion is far from definitive, said Bob Carr, a South Florida archaeologist hired by the project's developers to investigate the find. The nails are of a type used in the early 20th Century, a fact that may date the graves to that period.