Archaeologists and forensic experts in Guatemala have made a grisly discovery among the ruins of an ancient Maya city, Cancuén.

In explorations during the summer, they found as many as 50 skeletons in a sacred pool and other places, victims of murder and dismemberment in a war that destroyed the city and, it seems, served as a beginning of the collapse of the classic period of the Maya civilization. The precipitous decline of the Maya is one of the enduring mysteries of American archaeology.

As the scale of the massacre became apparent, the archaeologists called on Guatemalan forensic investigators for their experience with mass burials of modern war. The team, established in 1996 to excavate the mass graves from Guatemala's civil war, has also analyzed sites in Bosnia, Kosovo and Rwanda.

Arthur A. Demarest, an archaeologist at Vanderbilt University who directed the excavations, described the discovery yesterday in an announcement by the National Geographic Society and in an interview by telephone from Guatemala City.

"This is probably the most important thing I've ever discovered," said Dr. Demarest, who has explored Maya ruins since the 1980's.

In a gruesome departure from what had been normal Maya warfare, he said, the conquerors - not yet identified - did not spare the city to rule it as a vassal state.