The most probable explanation is the one identifying St. Petersburg’s terrorists as a group perpetrating jihadist actions in relation to the Muslim Chechen guerrilla warfare - as happened in Volgograd, Moscow and in other cities.
The death toll of St. Petersburg’s attack currently stands at 11, with forty people injured.
The attacker has been identified as a naturalized Russian national born in Kyrgyzstan.
The timing of the attack was perfect: Vladimir Putin was in town to meet with Belarusian President Lukashenko and attend a Forum on media.
It is as if the attack were "dedicated" to Vladimir Putin.
Probably it has also to do with the crisis of image caused by the arrest, a few days ago, of the opponent Alexei Navalny.
When a country is under a crisis of "image", regardless of the causes, jihadist terrorism steps us or delays the already planned action to convey an image of weakness and marginalization of the target country’s ruling classes.
As always happens in these cases, the explosion was caused by an improvised explosive device, so as to avoid passages and transfers of more advanced weapons or explosives that the intelligence services monitor and trace.
Indeed, allegedly the explosive devices were two, with 200 or 300 grams of TNT - which can be homemade.
As already mentioned, the suicide bomber is a Kyrgyz, certainly in contact with the Syrian jihad, identified - according to the Russian intelligence services - as Akhbarjon Dialjlov, born in Osh in 1995.
Therefore the most reasonable options are two: either the attack - as often happens with the "sword jihad" - shows the return to Russia of the Chechens who fought with Daesh-Isis, or it is the start of an autonomous campaign of Central Asia’s jihad against the only power which fights effectively against the Islamism of the "holy war" in Syria.
Or it is a punishment for the war in Syria or the start of Central Asia’s jihad, linked to the Chinese one.
A pincer movement by jihadist terrorism, so as to encircle Russia and prevent it from entering successfully the anti-jihadist fight in the Middle East or, in the future, in Asia.
Obviously the two Islamist terrorist operations are not clearly separable.
Hence, in all likelihood, after the Muslim war in Chechnya, the jihad plans to set fire to the whole Southern border of the Russian Federation.
It would be the end of Russia’s Eurasian project, as well as the start of its isolation and - hence - of its economic and strategic crisis.
However, considering the extent of the attack perpetrated in St.
Petersburg, we must assume there is a direct connection with Syria’s jihadist Islamism, designed to "punish" Russia for its specific engagement in Syria in favour of Assad, as well as with other grey areas of the Asian jihad: Kazakhstan, Turkmenistan, Uzbekistan, Kyrgyzstan and the radical Islamic network of Turkmen in Xinjiang.
In this context, we can assume a massive return of Chechen jihadists from Syria and the Daesh-Isis territory, who start again to "operate" against Russia and, later, against the Asian countries allied with Russia: China, the Central Asian republics and probably also Iran.
In fact, a few days ago the Islamic State released a video of threats against Iran, with jihadist attacks which will be organized by a new Isis-Daesh "division", the Salman al-Farsi, named after a "comrade" of prophet Muhammad.
Currently Daesh is operating in the Iraqi province of Diyala and it is moving towards Anbar.
The next points triggering the Caliphate’s jihad will be the Lebanon - behind Damascus and hence encircling Syria - Central Asia and Iran. Later the jihad will shift to Xinjiang.
A real encirclement of the Russian Federation, which has been correctly identified by Islamist terrorists as their only credible enemy.
Therefore, during phase 2 of Daesh-Isis, all the above stated countries, allied with Russia, will be hit. According to jihadists, if Russia is hit, the military and security response of its peripheral allies will be weakened.
The repression of the Chechen jihad had been strong and definitive in 2014, just before the Olympic Games in Sochi, a well-known primary target of the anti-Russian jihad.
Furthermore, the great campaign against Chechen Islamists (but with militants in Chechnya coming from all Central Asian republics) included a very secret "covert plan" to send terrorists to the Middle East, with the solemn promise not to go back to Russia any longer.
It is by no mere coincidence that the terrorist attacks in Volgograd were not followed by other attacks, as it could have been predicted initially.
It is also well-known that the jihadists coming from the Russian Federation and the former Soviet Islamic republics accounted for the largest "national" faction in Isis-Daesh.
Hence the attack perpetrated in St. Petersburg is designed to show
that: 1) Putin's strategy of fighting Daesh in Syria to avoid terrorists returning to Russia has failed; 2) there is unity of purpose and strategy between the Syrian-Iraqi jihad of Daesh and Central Asia’s Islamist terrorism up to Xinjiang; 3) the ultimate goal is to "punish Russia" for its war in Syria - and later other countries will be punished, namely those surrounding the new centers for spreading the "Islamic State".
Jihadist terrorism affects the States bordering on those countries that Islamist militants want to hit directly.
Suffice here to recall the continuous attacks in Iraq and Turkey which preceded the creation of the so-called "Caliphate" in Iraq and Syria.
Hence, probably if there is no extensive and decisive action by the Russian intelligence services, attacks will continue so as to instill in Russian decision-makers the "scare" to fight jihad for fear of a sequence of terrorist "punishments".
But certainly Vladimir Putin will not be scared.