Thousands of people from the Russian Federation, mostly from the Caucasus region, had traveled to Syria to join Islamist groups
. Many have developed reputations for being fierce, capable fighters.
The Russian government, like those in France, Belgium and the UK, have long feared that its own battle-hardened citizens could return to wreak havoc.
While Russia's role in the Syrian civil war has been interpreted by some in the West as a cynical and self-interested move to prop up the Assad regime -- US President Barack Obama described Putin's involvement in Syria as being
"not out of strength, but out of weakness because his client Mr. Assad was crumbling" -- it was sold to the Russian people as a necessary counterterrorism operation.
Some analysts believe that while Russia's activity in Syria has successfully killed many potential terrorists, it may also have motivated many others to strike back at Russia.
Indeed, ISIS claimed in 2015
that this was the motivation for the planting a bomb on a Russian passenger jet in October 2015, killing 224 people.
The Syrian connection isn't Russia's only terror concern.
A long-simmering, separatist and Islamist insurgency in the northern Caucasus region has repeatedly proved capable of staging large, devastating operations.
The world was shocked by events such as the Moscow theater siege in 2002
, which left 129 dead.
The Beslan school attack of 2004
killed more than 330 people, many of them children.
The last attacks before Monday's bombing in St. Petersburg were two suicide blasts in the southern city of Volgograd in December 2013.
The location of that attack, only a relatively short distance from the Caucasus, was at the time thought to show that terrorists no longer had the capability to strike Moscow or other high-profile targets. Clearly, this has been proved to be wrong.
Russia's security and intelligence services have in recent years dedicated huge effort, resources and expertise to containing and disrupting the terror threat from its southwestern flank.
As more details about the attack emerge -- at present no group or individual has claimed responsibility -- Moscow will be desperate to learn how anyone was able to slip past the country's formidable security infrastructure.