This story is being published jointly by VICE News and the Center for Public Integrity.

CHISHINAU, Moldova — The sample of highly-enriched uranium, of a type that could be used in a nuclear bomb, arrived here on a rainy summer day four years ago, in a blue shopping bag carried by a former policeman.

According to court documents, the bag quickly passed through the hands of three others on its way to a prospective buyer. It was not the first time such material had passed through this city, raising international alarms: It had happened twice before. And mysteriously, in all three cases, spanning more than a decade, the nuclear material appeared to have the same origin — a restricted military installation in Russia.

This news would quickly reach Washington. But that day, the first to pick up the blue bag was the wife of a former Russian military officer, who handed it off to a friend while she went shopping in this former Soviet city's ragged downtown.

Not long afterward, a 57-year old lawyer named Teodor Chetrus, from a provincial town near the Ukrainian border, retrieved it and brought it to a meeting with a man named Ruslan Andropov. According to an account by Moldovan police, the two men had, earlier in the day, visited a local bank, where Chetrus confirmed that Andropov had deposited more than $330,000 as an initial payment.

Andropov next examined the contents of the bag: a lead-lined cylinder, shaped like a thermos. It was meant to be the first of several shipments of highly-enriched uranium totaling 10 kilograms (22 pounds), a senior investigator here said. That's about a fifth of what might be needed to fuel a Hiroshima-sized nuclear explosion — but almost enough to power a more technically-advanced "implosion-style" nuclear bomb.

But then, abruptly, Chetrus' participation with this group of shadowy characters in the illicit sale of nuclear explosive materials — the stuff of nightmares at the CIA, the Pentagon, and the White House — went awry.

Andropov turned out to be working with Moldovan police, who were monitoring communications between those involved, with advice from the US embassy in Chishinau.

On June 27, 2011, they swooped in. Photos of the arrests show a policeman in a ski mask holding a Kalashnikov while Chetrus knelt on a sidewalk in front of the bank. He would eventually be sentenced to five years in prison.

Chetrus' arrest ended one of four attempts in the past five years by Moldovan residents to smuggle dangerous nuclear materials into the hands of unscrupulous buyers. But his capture did not ease the concerns of Western intelligence services.

No one in the West knows exactly who has this nuclear explosive material, and where they may be.

Instead, it stoked them, because the resulting international probe into the case has sparked fresh, and previously unreported, worries that thieves inside of Russia somehow made off years ago with a full bomb's worth of highly enriched uranium. Western spies fear the thieves have been doggedly looking for buyers for the past sixteen years, by repeatedly dangling in front of them identical, genuine samples of that highly valuable material.

Five current or former US officials who have tracked nuclear smuggling, and who declined to be named because this assessment is classified, said it is now a consensus view within the intelligence community.

But no one in the West knows exactly who has this nuclear explosive material, and where they may be.

It's a mystery that so far has stumped America's best spying efforts, in no small measure because the government of Russian president Vladimir Putin has refused to provide needed information on the case — or even to acknowledge that some of the country's nuclear explosive materials are missing.

Western concerns are based on a simple trail of evidence that officials have until now kept secret:In each case those holding the uranium said it was part of a larger cache, available to a buyer for the right price. That claim, while unproven, is considered credible by experts who have studied the three incidents.

Confidential forensic analysis by US and French nuclear scientists has shown that these materials came from the same stockpile. Officials say they believe all were produced in the early 1990s at a sprawling Russian nuclear facility known as the Mayak Production Association, located in Ozersk, in the Ural mountains, roughly 900 miles (1,400 kilometers) east of Moscow. The facility, which produced the fuel for Russia's first nuclear warheads and for its naval nuclear reactors, is still one of the country's "closed cities," where access is tightly regulated.

The similarities between these three seizures make them the most worrisome unresolved instances of illicit trafficking in authentic, bomb-grade materials anywhere in the world, according to more than a dozen government officials and independent experts interviewed for this article, many of whom spoke on condition of anonymity due to the sensitivity of the topic.

While seven of those involved in the smuggling have so far been prosecuted in Bulgaria, France and Moldova, officials say they are just low-level members of a shadowy international ring with Moldovan and Russian connections, all working for a person or persons whose identity remains cloaked.

Intelligence professionals — who say they put the issue of nuclear smuggling near the top of all their priorities — explain that this is a hard target to hit. Their principal ambition is to catch the thief and the buyer, but so far they have seen only middlemen.

But evidence collected from the probes of these three incidents indicates that a weapons-grade cache of nuclear material has been "in the wild since the mid-1990s," a knowledgable US intelligence official said. It's widely thought to be no longer in Russia, and to possibly have passed through multiple hands, the official added, explaining that the 2011 Moldovan case is what helped solidify this assessment.

The basis for international worry, several officials explained, is the potential for all or part of this nuclear-materials cache to wind up in the hands of a terrorist buyer who could transform it into a viable weapon, using technical information about nuclear bomb designs that has leaked long ago into the public domain.

The FBI has privately discounted Moldovan claims that radioactive materials seized in more recent smuggling incidents here were being sought by the Islamic State terrorist group. Still, American worries about the 2011 Chishinau case were heightened by the presence in the Moldovan capital at the time of the deal of a potential buyer from Sudan, where Al Qaeda tried to obtain some uranium in the early 1990s and remains active, officials here and in Washington said.

With so many nuclear explosives held by governments around the world, US officials have long worried about the possibility of a terrorist-engineered nuclear or radiological blast within the United States. Multiple federal agencies have held almost 1,400 drills in cities around the country over the last decade to train local police and emergency personnel in how to behave after such a nightmare unfolds, according to a spokeswoman for the National Nuclear Security Administration.

Asked at a March 2014 nuclear security summit in the Netherlands whether he thought Russia's assertive foreign policy was the number one threat to the United States, President Obama replied that: "I continue to be much more concerned, when it comes to our security, with the prospect of a nuclear weapon going off in Manhattan."

According to a 2004 Department of Homeland Security guidebook to disaster response, even a relatively small nuclear detonation — comparable to 10,000 tons of TNT, or about half the force of the blast that levelled Nagasaki – would kill hundreds of thousands of people, contaminate 3,000 square miles (7,800 sq km), and cause billions of dollars in damages, while leaving an urban area a mile (1.6 km) in diameter a smoking wasteland.

Besides making the arrests in 2011, Moldovan police detained three people who they said were trying to smuggle depleted uranium in August 2010, and last year the FBI helped investigate a group that tried to smuggle low-enriched uranium — neither of which can be used in a nuclear bomb. This year, further arrests were made in a case involving cesium, a radioactive, but not explosive, material.

Experts say Moldova's repeated role in nuclear smuggling is unsurprising, since cross-border crime is much more prevalent in poorly governed or fractured states.

Roughly the size of Maryland with about two-thirds the population, it is one of the poorest former Soviet republics.

Filled with rolling fields and tiny villages, the country is squeezed between Romania and Ukraine and brushed by the Danube River. The capital of Chishinau, a brash and dusty place, shows signs of fast economic growth that has benefited only a sliver of its citizens. BMWs and Lexus sedans share the streets with hordes of tiny taxis and Soviet-era streetcars, and pensioners line the sidewalks peddling soaps, samovars and women's underwear.

Since 1992, its territory has been split into two ethnically separate regions, dominated one by Romanian and the other by Russian and Ukrainian speakers. Russian troops have been stationed for decades in the second of these regions, known as Transnistria, a sliver of land on the eastern bank of the Dniester River, over the opposition of the central government.

Like other fragments of the former Soviet empire occupied by Russian troops, Transnistria is a haven for smugglers, particularly of cigarettes, arms, and prostitutes. It has its own flag, displaying a hammer and sickle, but isn't recognized as a country by any member of the United Nations, including Russia.

The Transnistrian capital, Tiraspol, is where Galina Agheenco – who picked up the blue bag containing the uranium from the former policeman and passed it to a friend — lived with her husband Alexander, 58, a mustachioed Russian former military colonel, according to officials here. An English-language slide presentation about the incident prepared by the Moldovan Ministry of Internal Affairs calls Alexander the "leader of the criminal group" involved in the nuclear smuggling incident.

His ambition, a Moldovan Supreme Court ruling in May 2014 said, was to sell a total of one kilogram of highly-enriched uranium for roughly $36 million, in a deal plotted on Skype, on mobile phones, and in emails — many of which turned out to be monitored by the government. The actual material offered prior to the police raid was one-hundredth of that amount.

But Col. Gheorghe Cavcaliuc, a soft-spoken, young Moldovan police official who heads the special operations division, said in an interview here that efforts by the police to learn more about Alexander's activities and connections have been stymied. An arrest warrant for him is still unfulfilled, five years later, and officials here say they heard he fled from Transnistria to Russia. Attempts by the Center for Public Integrity to obtain his response to the allegations against him were unsuccessful.

"We sent several requests to the Russian Federation for information about him, but we didn't get any answers," Cavcaliuc said.

Washington hasn't fared any better. The U.S. Embassy here "does not maintain liaison relationships or active, ongoing contacts with Transnistrian law enforcement and/or security service personnel," a December 2009 State Department cable released by Wikileaks said.

Galina Agheenco, whose Lexus GS330 car had Transnistrian plates, was detained on the day of the incident and served three years in prison. But the former policeman who brought her the uranium, and was charged in the case, returned to Transnistria when he was released by a court pending trial, defying a judge's order, according to the Moldovan police report. Chetrus, meanwhile, was freed from prison last December and is appealing his sentence.

The two other cases involving identical samples of nuclear explosives — in France and Bulgaria — also had Moldovan connections, according to investigators here.

The 2001 Paris case arose from a tip given to the police there that a 36-year old Frenchman with a criminal record, Serge Salfati, was trying to find a buyer for 39 kilograms (66 pounds) of highly-enriched uranium — more than enough for a skilled bomb-maker to produce a nuclear explosion. He was using genuine samples weighing a total of 5 grams as a lure.

A special police squad checked for radiation in Salfati's apartment and garage, but found nothing. Their detectors then got a hit from a van he used, and so they arrested him and seized a lead container containing the samples.

The plane carrying the uranium to Paris flew to Charles de Gaulle airport from Chishinau, said Ionel Balan, Deputy Director of Moldova's National Agency for Regulation of Nuclear and Radiological Activities, in an interview here.

The Bulgarian case, two years earlier, arose when a man driving over a bridge at a Danube River crossing to Romania aroused the suspicions of a border guard, who searched his vehicle. The guard found a receipt, written in Cyrillic, for the purchase of "uranium 235," and then, after pulling apart an air compressor in the trunk, found a lead container inside with that label on it. The man, Urskan Hanifi, told police he bought the material in Moldova and was headed back there after failing to find a buyer for it in Turkey, according to media accounts and a US government report.

Cavcaliuc said he is convinced that a single group stands behind each of the three smuggling cases, and that a larger cache of material could be hidden in Transnistria. "In all three cases, there was the same container, the same chemical components [of the uranium] and traffickers from the same country, Moldova," he said.

But no one knew any of this immediately. When the lead canister seized in 2011 was initially brought to the Moldovan government's rudimentary police laboratory, Balan, a biochemist, expected it to be a hoax and so he handled it without gloves or a smock. He found the inside wall had been coated with about an inch of paraffin wax, and inside it was a small glass ampoule shaped like a tiny harpoon, containing a blackish powder.

He used a snub-nosed radiation detector to take two readings, and then he consulted a dog-eared copy of a nuclear materials guide published by Los Alamos National Laboratory in the United States and used worldwide as a reference manual: "And immediately, I understood this was not simple or natural uranium, it was enriched uranium."

His readings also indicated some of it had decayed, "a clear indication that this sample was old and not fresh."

Word of this result quickly reached Washington and, shortly afterward, an unmarked private jet landed at the Chishinau airport, secretly carrying FBI agents. They scooped up the canister and its contents and flew them back to the United States.

The samples were taken to Lawrence Livermore National Laboratory, where US nuclear weapons have been designed since the 1950s and a group known as the Forensic Science Division specializes in analyzing foreign materials, using a classified library of radioactive particles collected by US officials and intelligence sources all over the globe.

According to Moldovan authorities, a preliminary report by the division, entitled "Results for Moldovan HEU Sample," concluded that the uranium was produced in Russia and eerily similar to the materials seized a decade earlier in France and Bulgaria.

They did not provide details, but US officials said the isotopic signature, along with other evidence, pointed directly at Russia's Mayak plant as the origin.

Patrick Grant, one of the lead Livermore investigators, declined to discuss the Moldovan case, but said in an interview that the findings in the Paris case "correlate very well" with those of the uranium seized in Bulgaria. In a 2014 textbook for nuclear forensic scientists, Grant and several colleagues wrote that the ampoules seized in Bulgaria resembled those used to preserve samples from specific production runs at Russian nuclear processing plants. Each of these plants, he said, might have dozens of such samples on its shelves.

Matthew Bunn, a nuclear security expert at Harvard who wrote a classified study about Russian fissile material stocks during the Clinton administration, said such samples would be relatively easy to steal. "You could easily imagine a room full of hundreds of samples...and someone sweeping them into a suitcase and walking out," he said.

The apparent age of the purloined materials is not reassuring. The Livermore team fixed the time of the Bulgarian sample production as October 30, 1993, plus or minus one month — a time when Russian political turmoil and economic problems had by many accounts seriously weakened security at the country's nuclear installations.

At some facilities, security guards and scientists alike were not paid, and morale plummeted. Moreover, "they didn't have seals, badge systems, or a computerized database" that showed how much explosive material they had and where it was, a US official said, speaking on condition of anonymity.

In 1994, a machinist at the Elektrostal Machine-Building Plant, a nuclear fuel production facility 36 miles (57 km) east of Moscow, told police he carried six and a half pounds (3 kg) of weapons-grade uranium out of the front gate, hidden in a pair of protective gloves. He gave the material to a relative, a butcher in St. Petersburg, who stored it in a jar in his refrigerator while he and two friends – a pipelayer and an unemployed man – hunted for buyers at open-air markets.

That same year, a Elektrostal metalworker named Vladimir Luzgachev smuggled out another 3.7 pounds (1.7 kg) of enriched uranium in a bag of apples. He was not arrested until June 1995, when Russia's Federal Security Service learned of his efforts to find a buyer.

Both of those episodes occurred several years after the Federal Security Service arrested a group of nuclear workers for involvement in the theft of 41 pounds (18 kg) of nuclear explosive material from an unnamed facility in Chelyabinsk province, where the Mayak Production Association and three other major plants are located.

Viktor Yerastov, then chief of the Nuclear Materials Accounting and Control Department for the Ministry of Atomic Energy, said in the Winter 2000 edition of Russia's Yaderny Kontrol (Nuclear Control) magazine that if successful, that theft "could have inflicted a significant damage to the state." A 2002 CIA report to Congress separately quoted him as saying the amount stolen was "quite sufficient material to produce an atomic bomb."

Washington's anxieties about a potential radioactive "dirty bomb" or nuclear blast on US soil have always been centered around the risk that explosive materials – more than a bomb's mechanical workings — could fall into the wrong hands. "In the nuclear business, it's all about the materials," said Anne Harrington, deputy administrator at the National Nuclear Security Administration, in an interview last year. "You can make widgets, pieces and parts, but without the material you don't have an improvised [nuclear] device."

Nuclear security "has faded to a tertiary issue" under Putin.

Although roughly two dozen countries have enough nuclear explosives to make a bomb, Russia's materials have long been the chief Western concern. Of the roughly 20 documented seizures of nuclear explosive materials since 1992, all have "come out of the former Soviet Union," Harrington separately told the Senate Armed Services Committee's Subcommittee on Strategic Forces in April 2015. "We see a lot of former Russian military, former Russian intel...involved in nuclear trafficking," a US intelligence official said in an interview.

Officials say that's why Washington has spent about $4 billion over the past 25 years to help that country tighten control of the weapons-usable materials inside its vast nuclear complex. Russian nuclear facilities have made progress, they say, particularly in improving training for security personnel, installing new physical barriers and upgrading related sensor technology. New nuclear security regulations came into effect in 2012, and a civilian oversight group was created to ensure their implementation.

But a senior intelligence official from the Bush administration who retains security clearances said that he was still worried about material stolen decades ago that may be "sloshing around" outside the walls of these facilities. "The real concern is that the material got out of these sites before we paid much attention" to securing them, he said in an interview, speaking on condition he not be named so he could discuss classified analyses.

This anxiety has been sporadically acknowledged by intelligence officials in the past decade. "We find it highly unlikely that Russian authorities would have been able to recover all the material reportedly stolen," a National Intelligence Council report concluded in 2005, according to an excerpt read by then-Sen. Jay Rockefeller of West Virginia at a hearing of the Senate Select Committee on Intelligence in February 2005.

Rockefeller asked then-CIA Director Porter Goss whether enough had vanished from Russia's stockpile to build a nuclear weapon. "There is sufficient material unaccounted for so that it would be possible for those with know-how to construct a nuclear weapon," Goss responded. Rockefeller also asked if Goss could assure the American people the missing material was not in terrorist hands. "No, I can't make that assurance," Goss said. "I can't account for some of the material so I can't make the assurance about its whereabouts."

In November 2002, a senior Russian nuclear and radiation safety official, Yuri Vishnevskiy, affirmed that small quantities of nuclear materials, including highly-enriched uranium, had indeed disappeared from nuclear facilities. But Russian officials have been increasingly tight-lipped since then.

Former CIA director George Tenet, in his 2007 memoir, said that after hearing Al Qaeda was trying to buy Russian nuclear devices in 2003, an Energy Department intelligence official went to Moscow to seek information about "reports we had received of missing material." But the Russians refused to provide details, Tenet wrote, and "in the final analysis, it was still a game of spy versus spy."

To overcome some of this distrust, U.S. officials tried the following year to draw Russia into joint analysis of fissile materials seized in the Bulgarian incident, but had only limited success. Scientists at Livermore shared a half-gram of that highly-enriched uranium with Russia's Bochvar All-Russia Scientific Research Institute in Moscow, and paid them $50,000 to do an independent analysis.

According to a report by Michael Kristo, a chemist at Livermore, Bochvar scientists "confirmed the analytical results" reached at his laboratory, including the fact that the sample originated at a nuclear fuel reprocessing facility. But Bochvar did not agree with Livermore that this meant it came from the former Soviet Union, and instead claimed "it could have been produced by any nuclear state possessing the appropriate facilites," Kristo wrote.

"They're very guarded and sensitive about the possibility that anything is missing," a former senior Obama administration official said in a recent interview, echoing comments from many others in Washington. "They never told us" whether they investigated the 2011 Moldovan case or what they found.

The 2011 version of an annual CIA report on Russian nuclear security practices — the most recent one completed — reaffirmed that "we judge it highly unlikely that Russian authorities have been able to recover all of the stolen material," and added that large uncertainties exist about more recent thefts and the current state of Russia's safeguards.

Under Putin, Russia has steadily cut back its overall nuclear security cooperation with the United States, arguing that it no longer needs Washington's financial or technical assistance to safeguard its own fissile material stockpile. "It just faded to a tertiary issue under Putin," Michael McFaul, the US ambassador to Russia from 2012 to 2014, said in an interview. This year, for the first time in its budget proposal to Congress, administrators at the National Nuclear Security Administration shifted all Russia-related nuclear security expenditures to other purposes.

Officials with Rosatom, the state-owned corporation that runs Russia's nuclear energy and weapons plants, declined to be interviewed for this article. But Kirill Komarov, first deputy director of Rosatom, spoke briefly to a reporter for the Center for Public Integrity at Moscow's AtomExpo nuclear exposition in June.

Asked whether a cache of stolen Russian nuclear materials might be held by someone with ill motives, Komarov was dismissive, calling it "a question out of spy plots."

"You know very well that a very operational system of controlling nuclear materials has been established worldwide -— none of them are out of control," Komarov said, adding that these materials are not passed around like a box of matches among smokers. "Their movements are always strictly controlled," he said.

Vladimir Rybachenkov, a former counselor on nuclear issues at the Russian Embassy in Washington and now an advisor to the Russian Foreign Ministry, similarly dismissed fears that there were caches of Russian-made nuclear explosive materials that smugglers were dipping into to peddle on the black market.

"Many things are being invented, you know, kind of illusions," Rybachenkov said. "People like journalists like to write about things that they don't know for sure. So it's rumors — rumors and nothing more."