The 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, said Aurimas Navys, a former officer at Lithuania's State Security Department. 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 said, to identify such weaknesses and the individuals who might exploit them on behalf of Russia. Navys reckoned 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.

Russia pulled that off with help from supporters in Ukraine, many of whom had been cultivated by Russia's intelligence agencies. Kremlin supporters in Ukraine's military bureaucracy in Kiev proved especially damaging, Navys said. They stalled Ukraine's response to the seizure of its territory.

Ukraine had failed to search hard enough for Russian assets in its midst, said Raimundas Karoblis, Lithuania's defense minister. "We now, after Ukraine, have learned the lessons," he said.

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 belong to Russia. Troublingly, that was Russia's rationale for annexing Crimea.

The Baltic states reckon that to thwart a destabilization 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."

Consider the following hypothetical, said Stephen Flanagan, a specialist on Eastern Europe at America's National Security Council during Russia's offensive in Ukraine. A rumor 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 troops. At least two Russian security agencies operate in the Baltic states, Flanagan said.

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 online. In April, for example, a bogus report asserted that Karoblis had been arrested for accepting a bribe to promote U.S. interests.

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 hairstyles. Such things often mean little in and of themselves, but officials say tracking them is useful.

Polls suggest that few Balts believe life would be better under Russian rule. As a Vilnius Uber driver put it: "For us, Russia is an animal."

Russia, however, has ways to recruit foreign helpers who are not necessarily believers. In eastern Ukraine, for example, the GRU (Russia's military intelligence agency) has offered heads of 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 tends to recruit indirectly in the Baltic states, offering inducements via local NGOs 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 back home.

Russian agents approach Balts making these runs and offer money in exchange for help. Edvinas Kerza, Lithuania's vice-minister of defense, said his country therefore works hard to identify Lithuanians engaged in this hustle.

Russia also recruits by blackmailing visitors who accept the advances of beautiful women, Kerza said.

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. Kerza said Lithuania checks to see whether applicants for a government job have relatives in Russia.

Were Russia to invade the Baltic states, local Kremlin supporters could no doubt hamper resistance. But Anders Fogh Rasmussen, a former NATO chief, said Russia is more likely to step up efforts to stoke divisions and undermine trust in Baltic democracies.

On that, many Baltic officials are cautiously optimistic. Resistance to the Kremlin narrative of a decadent, fascist West is stronger in the Baltic region than in poorer eastern Ukraine. And even there, Russia is so unpopular that it has so far failed to secure control of a land corridor to Crimea.