Hours after a deadly bombing in St. Petersburg, members of the special services knocked on the door of a house in Kyrgyzstan's city of Osh, more than 2,000 miles away.
The leafy and socially conservative bazaar city of Osh, on the edge of Central Asia's Ferghana Valley, was until a few years ago the home of the suspected bomber — Akbarzhon Dzhalilov, who turned 22 only two days before Monday's attack.
It is economically struggling cities and towns like Osh that swell the vast ranks of mainly Muslim migrants from Central Asia seeking their luck on Russia's labor market.
And of the hundreds of thousands who make that leap from countries like Kyrgyzstan, Tajikistan and Uzbekistan, many have fallen under the sway of recruiters from militant Islamist groups.
Security service officers in Osh are questioning Dzhalilov's parents as to what might have led their son to blow himself up on the St. Petersburg train.
Dzhalilov left Kyrgyzstan to live in Russia in 2011 — the year after violent clashes in Osh between the ethnic Uzbek and Kyrgyz communities left hundreds dead. The violence not only claimed mostly Uzbek lives, but it also destroyed an uncounted number of their businesses and shut them out of the city's public life. After those events, many Uzbeks left the country for good.
For most people in Central Asia, the circumstances that propel them to Russia are more straightforward. Unemployment is rampant across the region, and deep-set corruption limits opportunities for the industrious to create their own wealth. Jobs as construction workers and janitors abound, but the conditions are poor and the monthly pay typically is in the low hundreds of dollars.
At the start of 2016, there were more than half a million people from Kyrgyzstan, around 850,000 Tajiks and 1 million Uzbeks living in Russia, according to officials in Moscow. Unofficially, the figure is likely much greater. Visa-free travel for most Central Asians — a legacy fringe benefit of the former Soviet Union — has always made getting to Russia relatively easy. Securing permanent residence is more complicated.
For many poorly educated, financially struggling and lonely men condemned to an existence of abuse in the workplace or at the hands of police in search of bribes, the sense of solidarity to be found in religious associations is a valuable consolation.
The unlucky ones drift toward extremist groups propagating messages of violence. Some are drafted by recruiters; others find their way there through social media.
The most malleable eventually become fodder for militant Islamist groups in the Middle East.
While war in Syria and Iraq has helped boost the number of volunteers to militant organizations, Noah Tucker, the senior editor for Radio Free Europe/Radio Liberty's Uzbek Service, said recruitment among ethnic Uzbeks actually predates the current stage of the conflict.
"Already before the Syria conflict was really heating up, the al-Qaida affiliate, the Islamic Jihad Union based in Pakistan, was targeting Uzbek-speaking migrants working in Russia ... particularly trying to target them based on resentment toward the way that they were treated," Tucker said.
Estimates on how many people from Central Asia have gone that route vary wildly. Officials in Kyrgyzstan have at various junctures estimated the number of citizens from their country enrolled in militant groups at around 500 — the bulk are said to be from the ethnic Uzbek community. Tajikistan and Uzbekistan offer even larger numbers for their own citizens.
Security experts caution against taking such evaluations at face value.
"Ultimately, we do not know how many have traveled. Those who give estimates are not transparent about how they reached their conclusions. So it is difficult to ascertain their accuracy," said Edward Lemon, a research scholar at Columbia University, specializing in radicalism in Central Asia. "There are still more questions than answers regarding how many Central Asians have gone to Syria and Iraq."
Central Asian governments as a rule speak about the dangers of terrorism in vague generalities, dwelling in sensationalist terms on the scale of the threat, while failing to distinguish between various groups.
Getting to the bottom of the St. Petersburg attack will require a nuanced understanding of the distinctions about which nationalities and communities are represented in which groups.
In the last two years, Kyrgyzstan's security services have repeatedly said their country has been made the object of numerous plots from the Islamic State group, although they have provided only muddled evidence to support such assertions.
As far as ethnic Uzbeks in Kyrgyzstan are concerned, they are predominantly represented in groups linked to the Nusra Front, which was formed as al-Qaida's branch in Syria, rather than to their avowed enemies in the Islamic State group.
"There seem to be very few ethnic Uzbeks from southern Kyrgyzstan fighting with (the Islamic State)," Tucker said. "There's very little propaganda that's produced to target them."
Tucker said the indiscriminate killing of civilians is a closer fit to the model favored by the Islamic State group. Nusra Front-affiliated Uzbeks instead traditionally try to limit their killing to the battlefield.
"The Uzbeks who fought in Aleppo, who were connected in one way or another to (the Nusra Front) ... they don't spend their time debating and rationalizing killing noncombatants, because they don't target noncombatants. ISIS guys do," Tucker said.
Then again, it is the Nusra Front — including its Uzbek contingent — that has been particularly badly battered by Russian-aided offensives in Syria.
Whoever ultimately is determined to be behind the St. Petersburg bombing, Russia could find itself once more counting the costs of its Syrian adventure.