For many years in the 1990s and 2000s, terror attacks hit Russia with alarming frequency, as the Chechen independence movement, which morphed over time into an Islamist terror group, targeted trains, planes and subway stations.

But outside Chechnya and the other restive republics of the North Caucasus region the number of attacks has decreased dramatically in recent years. Monday’s bomb on the St Petersburg metro was the worst act of terror outside southern Russia since a 2011 suicide attack at Moscow’s Domodedovo airport, which killed 37 people.

On Tuesday morning a Kyrgyzstan intelligence agency said it had identified a Kyrgyz-born Russian citizen as the suspect in the St Petersburg blast.

News agencies reported that the suspect had lived in Russia for six years and was from the city of Osh in southern Kyrgyzstan. There are hundreds of thousands of Central Asians living in Russia, who often work on construction sites in poor conditions, sending the money they earn to their families back home

No group has taken responsibility, but given the history of terror attacks in Russia, Islamist groups are likely to come under suspicion.

Early terror attacks by Chechens, such as the mass hostage taking at the Dubrovka theatre in Moscow in 2002 or at School Number One in Beslan in 2004, were accompanied by demands for a Russian withdrawal from Chechnya, rather than mass slaughter as their primary goal.

Later, the Chechen independence movement renamed itself the Caucasus Emirate, which sought to impose an Islamic state across the mainly Muslim North Caucasus region, and drew fighters from neighbouring republics such as Dagestan.

The Caucasus Emirate took responsibility for a 2009 train bombing, the 2010 Moscow metro bombings and the 2011 suicide attack at the city’s Domodedovo airport.

Doku Umarov, the self-styled emir of the insurgency movement, was killed in 2013, apparently after Russian security services sent poisoned food to him through middle men. Since then the group’s ability to strike at the heart of Russia has appeared to be on the wane.

Inside Chechnya, there have been reports of torture and of punitive house burning of families believed to have links to the insurgency. A renewed crackdown on any suspected militant activity in the run-up to the Sochi winter Olympics in 2014 and the departure of many militants to fight in Syria led to a weakening of the North Caucasus insurgency.

There is significant circumstantial evidence that Russian security services ignored suspected militants leaving the country for Syria, figuring they would be less of a threat outside Russia than inside. There are also reports of many Muslims from the former Soviet republics of central Asia being radicalised while working on construction sites in Russia and subsequently heading to Syria.

Russian security officials have said there are several thousands Islamist fighters from Russia and other former Soviet countries fighting in Syria.

Since Russia’s military intervention in Syria on the side of the Assad regime in September 2015, there have been a number of propaganda videos made by Islamic State militants promising revenge on Vladimir Putin and the Russian people. In October 2015, a plane travelling from Sharm el-Sheikh in Egypt to St Petersburg crashed, apparently after an explosive device was detonated on board. More than 200 people died, and Isis claimed responsibility.

Up until now, however, the only successful Isis attacks inside Russia have been in Chechnya and elsewhere in the North Caucasus. As well as targeting militant groupings of Russian origin in Syria, Russia has worked quietly to liquidate key players in the movement in Turkey. Assassins believed to be linked to Russian intelligence have also carried out a number of killings of figures believed to have links to the Chechen Islamist insurgency in Istanbul.

There is no evidence so far that Isis or North Caucasus militant groups were behind Monday’s explosion. There have also been a number of smaller explosions in St Petersburg in recent years that were linked to nationalist groupings, although the randomised slaughter on the metro does not fit with their usual methods.