T HE TIP was sent by a city tech worker: a single person could, in one fell swoop, disable almost every traffic light in Vilnius, Lithuania’s capital. It proved true, says Aurimas Navys, a former officer at Lithuania’s State Security Department. Mr Navys, who had received the tip despite his recent retirement, made sure the vulnerability was fixed. Lithuania and the other Baltic states, Estonia and Latvia, all NATO members, are scrambling, he says, to identify such weaknesses and the individuals who might exploit them on behalf of Russia. Mr Navys reckons that the defensive efforts of the Baltic states have multiplied tenfold since 2014. That was when Russia seized Crimea and, in Ukraine’s east, set off separatist fighting that continues today.

Russia pulled that off with help from supporters in Ukraine, many of whom had been discreetly cultivated by Russia’s intelligence agencies and Spetsnaz special forces. Kremlin supporters in Ukraine’s military bureaucracy in Kiev proved especially damaging, Mr Navys says. They deliberately stalled Ukraine’s response to the seizure of its territory. (Among the Ukrainians arrested for aiding Russia in 2014 was Ukraine’s army chief at the time, Volodymyr Zamana, though he was later freed.) Ukraine had failed to search hard enough for Russian assets in its midst, says Raimundas Karoblis, Lithuania’s defence minister. “We now, after Ukraine, have learned the lessons,” he says.

The Baltics are keen to avoid Ukraine’s mistakes. Recent remarks by Russian officials and Kremlin mouthpieces have highlighted the danger. As their propaganda has it, parts of Lithuania were gifts from Moscow in Soviet times and therefore rightfully belong to Russia. Troublingly, that was Russia’s rationale for annexing Crimea. The Baltic states reckon that to thwart a destabilisation campaign that Russia could launch, perhaps to support an armed attack, they should determine who might be susceptible to the Kremlin’s bidding. So the search is on for people involved in what officials call “Russian influence activities” as well as, more darkly, “sleeper cells” that could be activated from Moscow.

Consider the following hypothetical example, says Stephen Flanagan, a specialist on eastern Europe at America’s National Security Council during Russia’s offensive in Ukraine. A rumour spreads that an ethnic Russian girl in Estonia has been raped. A local pro-Kremlin motorcycle gang is told to wreak havoc. The Kremlin, which has asserted a right to protect ethnic Russians abroad, might then send in troops. At least two Russian security agencies operate in the Baltic states, says Mr Flanagan, who, before leaving government, studied the region’s defences against Russia.

Most of the suspects identified by Lithuania’s security agencies as part of this effort are classified as Kremlin “supporters”. People in this category might do things like pass along the fabrications against Baltic democracies that crop up relentlessly online. In April, for example, a bogus report asserted that Mr Karoblis had been arrested for accepting a bribe to promote American interests at Lithuania’s defence ministry. In one iteration, he was shown behind bars in a faked photograph. Lithuania has placed a smaller number of suspects in a different category: “potential doers”. Officials fear these people might take stronger action, perhaps rioting to discredit a democratic government.

Officials say they begin with tips, as well as hints of Kremlin sympathies. They might spot these in displays of military symbols or Vladimir Putin’s image, as well as in tweets, tattoos and even hairstyles. Such things often mean little in and of themselves, but officials say tracking them is useful. For starters, tallying even the seemingly trivial—reports of pro-Kremlin rants by drunkards, say—can reveal an important emerging pattern, says Tomas Ceponis, an analyst at Lithuania’s defence ministry. He is part of a team that records locations and other characteristics of pro-Kremlin or anti- NATO displays, graffiti and vandalism included.

Polls suggest that few Balts believe life would be better under Russian rule. As a Vilnius Uber driver describing past Russian domination puts it: “For us, Russia is an animal.” The average wage in Russia is lower than “a pension for a babushka in Estonia”, says Toomas Hendrik Ilves, Estonia’s president from 2006 to 2016.

Russia, however, has ways to recruit foreign helpers who are not necessarily devoted believers. In eastern Ukraine, for example, the GRU (Russia’s military intelligence agency) has offered heads of criminal gangs positions in a future Russian administration in exchange for help bringing it about. In Baltic countries the risk of arrest for Russia’s operatives is greater. Russia, then, tends to recruit indirectly in the Baltic states, offering inducements via local NGO s and the Socialist People’s Front, a Lithuanian leftist political party whose former leader was arrested last year on charges of spying for Russia.

A larger number of Balts, officials say, are recruited when they travel to Russia or its ally Belarus. Cigarettes and petrol are mostly cheaper there, so many people hop across the border, stock up, and sell the stuff once back home. Russian agents commonly approach Balts making these runs and offer money in exchange for help. Edvinas Kerza, Lithuania’s vice-minister of defence, says his country therefore works hard to identify Lithuanians engaged in this hustle. Most troubling, he says, are those who have access to sensitive information thanks to a job in government.

Russia also recruits by blackmailing visitors who accept the advances of beautiful women, Mr Kerza says. Another approach involves creating a legal problem for a relative living in Russia or Belarus. The Balt is then told that the charges will be dropped in exchange for spying services. Mr Kerza says Lithuania checks to see if applicants for a government job have relatives in Russia. Arrests of suspected spies are common in the Baltic countries.

Some fret that the search for fifth columnists can be taken too far. Eitvydas Bajarunas, the Lithuanian foreign ministry’s top official for hybrid threats, tells the story of men spotted in a wooded area near Vilnius. They were quickly rounded up by special forces but released soon after. They had been playing paintball. To process leads more effectively, a new intelligence centre is quietly being readied in Vilnius for operations to begin later this year. The Baltic Special Operations Forces Intelligence Fusion Cell, as it is called, is an initiative of Estonia, Latvia, Lithuania and Poland, with American help. They are being designed so that the analysts can be rapidly dispersed to work from vehicles, lest the headquarters be targeted in a conflict.