Among them was a square tablet depicting a procession. If genuine, its style would make it neither Roman nor Greek, like the rest, but even older, dating back nearly 5,000 years. Its appearance suggested it came from the ancient Sumerian city of Lagash, in what is today southern Iraq.
The police raid here, in March, was heralded as a rare success against the trafficking of antiquities, a crime that reached new levels as the Islamic State militant group took control of parts of Syria and Iraq, and destroyed and looted ancient sites. Yet it also highlighted the barriers that, dozens of art experts and officials in the United States and Europe say, hamper the fight against the illicit trade.
Laws around the world are weak and inconsistent, and customs enforcement can screen only a portion of what crosses international borders, according to officials and experts in trafficking.
Long-established smuggling organizations are practiced in getting the goods to people willing to pay for them, and patient enough to stash ancient artifacts in warehouses until scrutiny dies down.
Despite a near-universal outcry over the Islamic State's actions, few countries have shown interest in imposing new restrictions to curb the booming trade in antiquities, estimated to be worth billions of dollars a year.
"It's a broken system that ISIS or anyone else, whoever is next, can play into," said Donna Yates, an archaeologist at the Scottish Center for Crime and Justice Research at the University of Glasgow, referring to the Islamic State by one of its acronyms.
Officials still do not know how the artifacts ended up in Shumen or whether they passed through Islamic State territory. For every seizure like the one here, many other pieces are believed to reach dealers and buyers in Vienna, Munich, London and New York. Dealers exploit the legal trade in antiquities to move objects that have been looted for years amid the conflicts in Syria and Iraq, as well as Libya, Yemen and Egypt, officials and experts said.
Few objects have turned up so far that can be traced to the Islamic State's plunder. While the group is a relative newcomer to the looting, it has allowed it on an industrial scale in territory it controls, taxing excavations to raise money for the caliphate it declared in 2013.
That is why the antiquities seized here in Bulgaria - along with hundreds of pieces intercepted in Turkey, near the border with Syria, and at least one object seized in London and now stored for safekeeping by the British Museum - are viewed as part of a wave that experts expect to flood European and North American markets in coming years.
Satellite photographs have documented thousands of illegal excavations in Syria and Iraq, visible as pockmarks among some of the world's most important ancient ruins, like Mari and Dura-Europos in Syria. Tracking what has been looted from them, though, has proved difficult.
"We're faced with the largest-scale mass destruction of cultural heritage since the Second World War, and we're going to have to do something about it," said France Desmarais, director of programs and partnerships at the International Council of Museums.
The scale of the looting under the Islamic State has prompted many nations to try to stanch the flow, and the revenue, though it is hardly the group's largest source, given its trade in oil.
Just last month, for example, finance ministers from the 15 countries on the U.N. Security Council pledged to take steps to block the trade in oil and antiquities. In September, the State Department announced that it would offer a $5 million reward for information that disrupts the trade.
The International Council of Museums issued a new "red list" of categories of art and antiquities at risk of being looted from Libya in December. One for Syria was published in 2013, while the list for Iraq, first issued after the U.S. invasion in 2003, was updated last year.
Still, such efforts have done little to plug gaps in the enforcement of laws against trafficking or to stifle the insatiable demand of the antiquities market. In Germany, like some other countries, privacy laws protect buyers and sellers from scrutiny. The United States has no explicit prohibition on the sale of artifacts from Syria.
Some of the collectors willing to buy black-market artifacts are believed to be in Persian Gulf countries. But many others, experts say, are in the West.
"There appears to be an interesting geographic divide: Pre-Islamic objects go to Europe and North America, while Islamic art goes to countries of the gulf," said Markus Hilgert, director of the Museum of the Ancient Near East in Berlin, who is coordinating a research project on the illicit trade.
A recent report by the Washington-based Foundation for Defense of Democracies observed of customers in the West, "The main buyers are, ironically, history enthusiasts and art aficionados in the United States and Europe - representatives of the Western societies which I.S. has pledged to destroy."
Some objects have been shown fleetingly by middlemen in photographs on cellphones, using applications like WhatsApp.
Andreas Schmidt-Colinet, an emeritus professor at the Institute of Classical Archaeology at Vienna University, said he had seen images online purporting to be of funeral reliefs from Palmyra, the ancient city in Syria now controlled by the Islamic State. Images of other similar artifacts have also surfaced in catalogs offered by antiquities dealers, he said, but so far there is no evidence of their authenticity.
Since 2011, authorities in Turkey have seized 6,800 objects, a majority of them coins, and are holding them in regional museums until their origins can be determined, according to Necati Anaz, deputy manager of the International Center for Terrorism and Transnational Crime in Ankara. Anaz said the challenge was that many of the smallest objects could easily be carried in bags or in clothes.
In the United States, there has not yet been a documented case of a looted object linked to the Islamic State, according to officials from several agencies. The Immigration and Customs Enforcement bureau of the Department of Homeland Security repatriated more than 80 objects in 2015 in a ceremony at the Iraqi Consulate in Washington, but they had been taken during the U.S.-led war in Iraq.
The objects, including an ancient limestone sculpture of King Sargon II, a ruler of the Assyrian Empire in the eighth century B.C., had been seized by customs agents in New York in 2008 after they were shipped from a dealer in Dubai, United Arab Emirates, to a private collector, who was not identified. The origin of the statue and its route from Iraq to New York are murky, even after seven years of investigation.
Many of the recently looted artifacts, antiquities experts say, are probably being kept in warehouses in or near Syria and Iraq until attention has waned.
"Damascus, Beirut, Amman - the first stops are all the same," said Matthew Bogdanos, a Marine Reserves colonel who led the search for objects stolen from Iraq's National Museum in 2003 and co-author of a book about the effort, "Thieves of Baghdad."
"Don't underestimate how patient the dealers with international connections are," he said.
After antiquities leave the Islamic State's territory, experts say they fall into the hands of smuggling and criminal networks that are practiced at trafficking people, drugs or other contraband from Syria and Iraq. "Once they hit these networks, they go everywhere, and they're very hard to track," said Brent Easter, a customs special agent in New York who investigates art smuggling.
Bulgaria, which is home to scores of ancient sites, has a notorious history as a conduit for smuggling, officials said. When illicit antiquities cross into Europe, it becomes easier for smugglers to create documents listing them as authentic discoveries that can be legally bought and sold.
"We are the most direct route from Turkey and the Middle East," said Bozhidar Dimitrov, director of the National History Museum in Sofia.
Vladimir Kaidzhiyev, a senior police inspector in Shumen, said the police had acted on information that a smuggling network was moving a shipment through the region. Early one morning in March, dozens of officers raided the shed and three other places.
They confiscated more than 9,000 items, including figurines, jewelry and coins that were 2,000 years old, as well as molds and other material that suggested some of them could be fakes. The 19 larger sculptures included a lion's head, a Gorgon relief and a funerary panel. Many were encrusted with dirt, suggesting they had been excavated recently, not taken from a museum.
"I've seen something like this only in Rome," said Kaidzhiyev, who typically investigates more common crimes.
The larger pieces had coded labels; the tablet, for example, was marked 12x. Three suspects were arrested, including the owner of the shed, Veysel Sanli, a Turk with citizenship in Bulgaria. They are free on bail while the investigation continues.
The prosecutor overseeing the case, Margarita Georgieva, said in an interview that the items had arrived from Turkey, but that investigators had not been able to establish their destination or provenance, which makes trafficking cases hard to prosecute.
She said Sanli claimed to collect as a hobby and to have acquired some of the items from a shop in Izmir, Turkey, though the shop specializes in reproductions, and the seized goods appeared genuine. She added that investigators had found photographs of more artifacts on Sanli's cellphone and on memory cards, suggesting they were being offered for sale in Germany.
"There are not many people who could afford these items," Georgieva said.
So far, Sanli has been charged only with possessing cultural property, a crime punishable by one to six years in prison. Reached through his Facebook account, he declined to comment.
The Roman sculptures have been identified as genuine, but uncertainty surrounds the tablet. Zdravko Dimitrov, from Bulgaria's National Institute of Archaeology, concluded that it was probably genuine. McGuire Gibson of the University of Chicago, a leading expert on Mesopotamian art, reviewed a photograph of the tablet and noted unusual features, like a smooth bore hole in the center, that suggested it could be a well-made reproduction. But he added, "If that were real, they could sell it for millions."
The State Department is discussing the case with Bulgaria.
For now, the items remain in storage in the regional history museum in Shumen. The museum's director, Georgi Maystorksi, expressed relief that at least they had not fallen into private hands, never to be seen or studied. "There are so few successes in this world," he said.