When Eston Kohver, an officer with Estonia’s security police KAPO, was detained by Russian FSB agents two years ago and charged with espionage, the outside world was stunned. According to evidence presented by KAPO, the FSB agents had simply seized Kohver from Estonia’s side of the two countries’ shared border. But Russia’s intention, it is now clear, was not to unmask a spy operating in their country. It was to signal to its agents that it leaves no spy behind.

With six Russian agents arrested by the Estonians and sentenced to jail, versus none by the Russians, the Russians had to made a drastic move to free get their man, Aleksei Dressen, back. Dressen, a veteran KAPO officer, had been arrested in 2012 and was serving a 12-year prison sentence when Kohver was seized.

“Most intelligence services make sure that they get their officers and even agents back,” notes Nigel Inkster, a former director of operations and intelligence at MI6, Britain’s foreign intelligence agency, who now directs the Transnational Threats and Political Risk at the International Institute for Strategic Studies in London. “Spy swaps are mostly a Cold War phenomenon, but they’re still characteristic of Russia as well as Israel.” By contrast, other intelligence services now often prefer to get their men released by means of quiet negotiation.

During the Cold War, the United States and the Soviet Union frequently traded their bloc’s spies on Berlin’s Glienecke Bridge, including the 1985 swap of 23 Western spies arrested in Warsaw Pact countries for four Warsaw Pact spies jailed in the West. Another spy exchange on Glienecke Bridge is the subject of Steven Spielberg’s recent movie Bridge of Spies.

Moscow’s problem was that it didn’t have a spy to offer in exchange for Dressen or its five other agents in Estonia—and at any rate Estonia was intent on keeping the Russian agents in prison. But faced with news media reports about Russian soldiers in Ukraine disowned by the own country, Moscow had to send the signal to its intelligence officers and agents abroad that it is safe to work for the Kremlin. (An intelligence officer is a career member of an intelligence agency, while an agent is a freelance spy reporting to an officer.) Moscow had to find an Estonian agent operating on Russian soil.

After failing to do so, the FSB seems to have settled on Kohver, a KAPO officer working not on espionage but one cross-border crime investigations. While the FSB claimed that Kohver had been seized on Russian territory, the international community believed Estonia’s evidence and roundly condemned Russia for the seizure.

But the international criticism didn’t bother Moscow. Russia’s mission to the EU responded to Europe with a statement saying “we cannot but interpret the loud statements on this case as an attempt to politicise the criminal case, the legality and validity of the judgement of which is beyond any doubt.” Kohver was swiftly sentenced to 15 years in jail. One year after his arrest, he was swapped for Dressen on a bridge over the River Piusa, which connects Estonia and Russia.

The problem with swaps is that they have to be seen as an exchange of equal-value agents. “According to local fashion, the propaganda surrounding every swap should reflect an opposition of Russia versus the West, as opposed to ‘small countries’ like Estonia,” explains Andrei Soldatov, a Russian investigative journalist specializing in the security services. “In the case of Kohver, Dressen was portrayed as someone who had penetrated not only KAPO but gathered information about ‘Western’ intelligence operations against Russia as well.” But since it was obvious that the rather average Dressen had only spied on Estonia, the FSB inadvertently upgraded KAPO to an equal adversary. (The FSB did not respond to requests for comment.)

Today, even though KAPO has Kohver back—he is even back on duty in his old job—Estonian police and security agencies are concerned. The five other Russian agents, who include two arrested last year and one this year, are serving their sentences, and Moscow has nobody to swap them for. For the FSB, the unfortunate reality is that Estonia spies much less on Russia than vice versa, and that the Estonians are comparatively more skilled at catching Russian spies. Since the highly embarrassing unmasking of the Ministry of Defence security chief Hermann Simm as a veteran Russian agent eight years ago, KAPO has perfected its counterintelligence skills and is today considered one of the world’s leading spy-catching agencies.

But on February 10 of this year, the FSB arrested an Estonian businessman named Raivo Susi while he was in transit at Moscow’s Sheremetyevo airport and accused him of espionage. Susi, who maintains his innocence, is currently awaiting trial. The businessman has even offered to post a $100,000 bail, which has been denied. And on 13 May, the FSB arrested another Estonian resident, Arsen Mardaleishvili, in St. Petersburg and charged him with gathering military information for Estonia.

Susi and Mardaleishvili’s arrests have strengthened Estonian suspicions that Russia is trying to force another spy swap. But with two agents in Estonian jails, Moscow needs another alleged Estonian spy in addition to Susi. “The Russians are not just catching other countries’ spies,” says Inkster. “They’re essentially also taking hostages.” That makes it extremely dangerous for any Estonian who could conceivably be accused of espionage to travel to Russia or countries allied with it. Last spring Estonia’s parliament introduced stricter rules on travels by officials with access to state secrets. “It’s not that they’ve been banned from travelling; we just ask them to notify us first,” explains KAPO Superindendent Harrys Puusepp.

There is, however, one piece of good news for Estonians worried about being arrested and used as spy swap pawns. As Inkster points out, the agency responsible for the “near abroad”, former Soviet republics, is not Russia’s SVR foreign intelligence agency—which employed two of the agents serving prison sentences in Estonia—but the FSB domestic intelligence agency. Notes Inkster: “FSB agents are not as sophisticated as SVR agents.” They’re much easier to spot.