While antiquities collectors and dealers have seen a crackdown on looted cultural property and a surge in repatriation in recent years, there’s at least one case where the sellers are taking a stand.

Days before Sotheby’s hosted its sale “The Shape of Beauty: Sculpture from the Collection of Howard and Saretta Barnet” on May 14 in New York, Greece’s Ministry of Culture sent the auction house a letter. It asserted that a circa-8th century BC bronze horse that was the catalogue’s cover lot was actually the property of Greece and should be returned immediately. Sotheby’s withdrew the lot, which had been estimated at $150,000 to $250,000, and the rest of the sale went ahead as planned.

The Sotheby’s catalogue page for the horse, which has since been removed from the website.

But on June 5, both the Barnet heirs and Sotheby’s fired back at Greece, filing a lawsuit in US District Court for the Southern District of New York, asserting that Greece had interfered in the sale “without lawful justification.” They are seeking declaratory judgment that the bronze horse was “acquired lawfully and in good faith by the late Howard Barnet 45 years ago and has been part of their collection ever since.” The request also seeks a ruling that Greece has no ownership rights and that Sotheby’s can “lawfully” sell the work.

Sotheby’s declined to comment for this story. The Ministry of Culture and Sports of the Hellenic Republic, the defendant named in the lawsuit, did not respond to a request for comment.

The Ministry of Culture sent Sotheby’s the letter on May 11, “one business day” before the auction was to take place, says the complaint. The letter “set forth no factual basis—and there is no factual basis—to establish that the Bronze Horse belongs to Greece. In fact, the Barnet Family is the true and lawful owner of the Bronze Horse.”

The Barnets purchased the horse in November 1973 for roughly £15,000, according to the court papers. The provenance says that it was sold at a public auction on May 6, 1967, in Switzerland at Münzen und Medaillen. The Barnets eventually acquired it from dealer Robin Symes who “very probably” acquired it at the Swiss auction, according to the provenance listed in Sotheby’s catalogue.

Symes and his partner, Christo Michaelides, dominated the international antiquities market for most of the 1980s and ’90s but were later exposed as key players in an international criminal network that traded in looted archaeological treasures. Symes was never formally convicted of antiquities trafficking (a British court convicted him only of contempt of court), but a mention of their names is enough to raise red flags in the trade.

However, Sotheby’s points out that at the time the Barnets acquired the horse from Symes in 1973, he was “a highly respected art and antiquities dealer in London. Decades later, Symes was accused of trading in looted antiquities.” Further, the court papers assert “the documented provenance of the Bronze Horse pre-dates Symes’ alleged acquisition of the item. Therefore Symes’ involvement with the Bronze Horse lends not support to Greece’s claim of ownership.”

Prior to offering the horse at auction, Sotheby’s “conducted an inquiry into the provenance of the Bronze Horse and, among other things, confirmed that the Bronze Horse was in fact included in the catalogue for the May 6, 1967 Münzen und Medaillen auction.”

They also note that the Greek ministry’s demand letter “provided no information as to when the Bronze Horse was discovered in Greece, when it was supposedly stolen, who stole it, the circumstances under which it was stolen, when it was removed from Greece, or by whom it was removed from Greece.”

The horse was flagged by Christos Tsirogiannis, an antiquities expert who investigates possibly illicit antiquities by scanning auction catalogues and searching for matches in an extensive photo archive. Though he has earned a reputation as a tireless sleuth, he has sometimes been referred to as a “thorn in the side of auction houses,” as one colleague put it.

On May 1, Tsirogiannis says he sent a letter to Francoise Bortolotti, a criminal intelligence officer at Interpol’s works of art unit. He provided a copy of the letter to artnet News. After flagging the sale date at Sotheby’s, he wrote:

“Please find attached the three images of a bronze Greek figure of a horse, of the Corinthian type, from the confiscated Symes-Michaelides archive. The same figure is to be auctioned as lot 4 in New York, by Sotheby’s at their 15/4/2018 auction.” After citing the provenance given in Sotheby’s catalogue, he writes “Please notify the American judicial authorities in New York, as well as the Italian and Greek police authorities as it is of paramount importance to examine ‘Münzen and Medaillen AG’ in Basel in order to be discovered the identity of the consignor of this bronze horse back in 1967, a valuable information which will eventually lead to the country where the object was discovered.”

Bortolotti could not be reached for comment.

Some experts, including the auction houses, argue that they don’t have access to the same information and photo archives that Tsirogiannis does. Sotheby’s points out in its complaint that under federal law, “the absence of proof that an antiquity has been lawfully exported does not and cannot constitute proof that the antiquity has been stolen in violation of foreign patrimony laws.”

When we asked Tsirogiannis in April why he doesn’t go straight to the auction house when he finds red flags, he said: “No—you don’t go to the trafficker to alert them.” To date, his research has contributed to restitution by museums including New York’s Metropolitan Museum of Art and Los Angeles’s J. Paul Getty Museum.

This past April, his work prompted the withdrawal of an Etruscan amphora, valued at $30,000 to $50,000, from a Christie’s New York auction.

Sotheby’s has invited Greece to present by May 25 any other information to support its claim that the horse is stolen property and cannot be legitimately sold. So far, Greece has not responded to Sotheby’s request, the court papers say.

Sotheby’s also points out that the Greek Ministry’s letter references a memorandum of understanding between Greece and the US that was entered into in 2011. According to the papers, the memorandum is “irrelevant to the Bronze Horse because it does not apply to an object that was indisputably outside of Greece prior to 2011.”

In addition to the declaratory judgement, Sotheby’s and the Barnet family are seeking interest, costs, and disbursement relating to the action, as well as attorneys’ fees.