The fact that a second bomb was found at another St Petersburg metro station indicates that Djalilov was likely not acting alone and was part of larger conspiracy.
There are two groups that have both the capability and the intent to carry out large-scale terrorist attacks in Russian cities. First, there are Chechen separatists who have mounted a wide range of terrorist attacks in Russia. The Russians have been waging wars with these separatists since the 19th century, but for obvious reasons Chechen separatist terrorism tends to be carried out by Chechens.
The other, more likely perpetrator, ISIS has plotted to target St. Petersburg in the recent past.
In February 2016, Russia arrested
seven alleged ISIS militants who were plotting attacks in St. Petersburg and Moscow. The group included Russians and Central Asians and a ringleader who had come from Turkey. Russian authorities said they had discovered firearms and a bomb-making laboratory.
The large number of "foreign fighters" from Russia and the Central Asian states who have joined ISIS compounds the ISIS threat to Russia.
Last year, Russian President Vladimir Putin estimated the number of fighters who had left for Syria and Iraq from Russia and the former Soviet republics at 5,000 to 7,000
The Soufan Group, a New York-based intelligence consulting firm that tracks foreign fighters who have joined ISIS, placed the number of fighters from Kyrgyzstan and Uzbekistan, another post-Soviet central Asian state, at 500 each.
ISIS has also relied
upon its Central Asian recruits to carry out attacks in the past. ISIS recruits from Kyrgyzstan, Uzbekistan and Russia killed more than 40 people at Istanbul airport in June 2016.
If the attack on St. Petersburg was carried out by ISIS relying upon at least one Central Asian recruit, it would provide further evidence of the expansive and multi-layered threat ISIS poses to Russia.
ISIS despises the Russian government for its support of the Syrian dictator Bashar al-Assad, and so it's no surprise that ISIS began targeting Russia in 2015, around the same time
that Russia first intervened in the Syrian civil war.
On October 31, 2015, ISIS bombed a Russian airliner carrying vacationing passengers from Sinai, Egypt to St. Petersburg, killing 224 people. ISIS celebrated the attack
both in its English language magazine Dabiq as well as in its Russian language magazine Istok. The attack carried out by ISIS' Sinai affiliate illustrates how wide-ranging the threat to Russia is.
As ISIS loses on the battlefields of Iraq and Syria, contingents of Russian ISIS fighters who survive may try and make their way home to foment additional terrorism on Russian soil. They must be stopped from possible re-entry.
In addition to continuing the aggressive campaign against ISIS in Iraq and Syria that began under President Obama and has been ramped up under President Trump, the international community must share with INTERPOL as many names of "foreign fighters" as possible-- including the names of the thousands of Russian ISIS recruits -- so that as the group's foreign fighters disperse from the warzones in Iraq and Syria, they can be arrested as they attempt to transit out of the region.
And given the estimated 30,000 foreign fighters
that ISIS has manged to recruit, Russia and the international community certainly have their hands full.
This piece has been updated to reflect the latest reporting on the St. Petersburg blast.