Archaeologists have been puzzled over the centuries-old skeletons of six young men whose bones were found face-down, hands tied, and covered with stones in a cliff-side cave on Spain's Gran Canaria island, and they are determined to figure the mystery out.

Evidence at the Caleta de Arriba site points to a violent death centuries ago which could pre-date the 15th-century Spanish conquest of the archipelago off northwest Africa.

"They are not buried in the strict sense ... This is a funerary practice that denotes very strong symbolic violence," explained Veronica Alberto, head of the excavation.

The bodies had been forcefully placed on the rocky surface and then had stones thrown on them, she told Reuters. Some had straps or bindings on their limbs.

"It could be an aboriginal burial, but because of the characteristics of the archaeological site we need to expand it and consider that they could be from other periods after the conquest, from the 16th, 17th, 18th centuries," Alberto added.

Without objects, to help establish the period, scientists are waiting for radiocarbon dating tests on the bones.

The archaeologists, who use security ropes while working on the steep terrain, said it was not a normal burial place because they only found men. All six had strong arms, suggesting they were involved in the same type of physical activity.