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Greece has ratified a complex deal for the return, over coming decades, of 161 striking ancient Greek artifacts from a U.S. billionaire’s collection after Athens conceded it had no evidence they’d been illegally excavated and exported.

The mostly marble works date from 5300-2200 BC, and the bulk of them are from the Early Bronze Age Cycladic civilization whose elegantly abstract but enigmatic marble figurines inspired artists from Pablo Picasso to Constantin Brancusi. Such pieces are highly prized by collectors and museums, which has spawned a wave of illegal excavations in Greece — and countless forgeries.

Greece’s parliament on Thursday approved the agreement with New York’s Metropolitan Museum of Art, a top Athens museum and a Delaware-based cultural institution to which they are being transferred. The works will return to Greece gradually from 2033-2048, after being displayed at the Met from 2023-2048.

Culture Minister Lina Mendoni described them as “masterpieces … of unique archaeological and scientific value” that Greece is getting without a messy court battle. Before going on display at the Met, 15 of the works will travel to Athens for a year-long exhibition starting November.

“They won’t return tomorrow … but they will (gradually) return,” Mendoni said during a parliamentary debate Thursday. “This collection was completely unknown to the ministry.”

But opposition lawmakers, and many archaeologists, charged that the agreement would whitewash the global trade in undocumented and potentially illegally excavated antiquities. They argued that the government should have fought a legal battle for their immediate return.

Little is known on the provenance of the 161 works from the collection of Leonard N. Stern, an 84-year-old pet supplies and real estate businessman and philanthropist. That means archaeologists can glean minimal useful information on their original use and significance. And a Greek Culture Ministry official told The Associated Press that the ministry has not yet examined the works’ authenticity.

The official was not authorized to discuss the matter with the press, and spoke on condition of anonymity.

Most of the pieces are the typical, broad-faced, flat white marble statuettes depicting nude women with folded arms — although there are some unusual types too. There are also marble bowls and vases, a terracotta frying-pan shaped dish and a pair of bracelets.

Mendoni said the ministry had no evidence that they were illegally exported from Greece. “We may understand that, we may sense it. We can’t prove it,” she told Parliament.

“A legal effort to claim the collection was estimated to have minimal chances of success, and would not have secured the return of all 161 antiquities,” she added. “And we want them all repatriated.”

The Cycladic civilization flourished in the Cyclades islands in the Aegean Sea during the 3rd millennium B.C. Its marble artifacts are admired for their abstraction, strong lines and white form. Originally, however, they were colored.

The government says a model similar to that employed for the Stern collection could be used for other significant Greek antiquities abroad.

“We want to attract the attention of other collectors, and secure more returns,” Mendoni said.

All antiquities found in Greece are by law public property, and officials regularly monitor auction houses and collections abroad for potentially plundered artifacts.

Athens has also long and fruitlessly lobbied to get back large sections of the 5th Century B.C. sculptures that originally decorated the Parthenon Temple on the Acropolis and are now in the British Museum in London.