Why Russian metro attack could have been much worse

John Sparks, Moscow Correspondent

It did not take officials at the Russian General prosecutor's office long to declare the St Petersburg metro blast a terrorist attack.

The explosion , which took place at 3pm local time on the line running through the central Sennaya Ploshchad station, was caused by one, relatively small device consisting of 200g-300g of explosive material and metallic shrapnel.

Contrary to initial reports in the Russian media, there was only one explosion but the national anti-terrorism committee, made up of officials from parliament and the security services, said an additional, unexploded bomb was later found in a carriage at Ploshchad Vosstaniya.

This device was much larger at 1kg and designed to look like a fire extinguisher. It now appears this tragic event could have been much worse.

Unsurprisingly, the Russian president, Vladimir Putin, said investigators would "prioritise terrorism" as they searched for the cause and told waiting reporters he had already been briefed by the head of the federal security service, or FSB.

The leader was in St Petersburg at the time, preparing for a meeting with the Belarusian president Alexander Lukashenko.

"Unfortunately, we have to start our meeting like this," he told waiting reporters. "I have instructed the city authorities to give all the required support to families of the victims."

The Russian state maintains a formidable security infrastructure - resources that Mr Putin will now direct at uncovering the culprits of an attack that has killed at least 14.

However, it is unlikely this explosion will have come as a surprise to members of the country's intelligence services.

As in other parts of Europe, Russian civilians and Russian cities are potential targets for so-called Islamic State.

We know that because members of IS have published videos online, in the Russian language, threatening revenge for Russia's role in Syria as Bashar al Assad's principle backer.

IS claimed responsibility for the deaths of 219 Russians travelling home from Egypt in an airliner operated by Metrojet in 2015.

The aircraft blew up 23 minutes after take-off from Sharm El Sheikh airport. Investigators later said a large bomb had been loaded in the luggage hold.

On Russian soil, two separate suicide attacks in the city of Volgograd took the lives of 34 in 2013.

The bombers claimed responsibility, in Russian, in the name of the "Caucasus Emirate" - an Islamic separatist group, but terrorist attacks in Russia have been relatively rare in recent years.

By means of explanation, analysts believe thousands of men active in extremist circles have travelled south in order to fight with IS and other groups in Syria and Iraq.

Mr Putin has justified his country's actions in Syria in this context - better to fight an estimated 7,000 extremists from the ex-Soviet republics in the Middle East, he says, than to wage war with them at home.

Still, Russia's expansive and highly risky foreign policy has created plenty of enemies - the sort of people who may wish to do him and his countrymen harm.

And the bombs on the St Petersburg metro system serve as a reminder to all that Mr Putin's Russia is vulnerable too.

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