Monday's explosion in the St. Petersburg metro looks to have been deliberate. Attacks on the metro have become a painful reality for Russia, which can add this incident to a long list of similar experiences.
Twisted subway cars, smoke and bloodied passengers: As details continue to emerge about Monday's explosion in St. Petersburg's subway that resulted in dozens of dead and wounded, the images are a grisly reminder of similar events from the recent past.
In Moscow in 2010, 41 people were killed in two subway explosions 30 minutes apart. The Islamist suicide attacks were carried out by women from Russia's northern Caucasus region.
Following the evidence
Moscow's subways have been targets since the mid-1990s.
Two attacks took place in 2004: A suicide bomber killed 40 in February of that year and eight more people were killed in August when a woman set off her explosives at a subway entrance.
Nearly all cases can be traced back to Islamists in southern Russia, especially from the breakaway province of Chechnya, where Russia waged two wars against separatists in the 1990s.
Attack in Russia's second city
Volgograd in southern Russia was targeted in 2013. Explosions in buses and at the central station killed 34 people. Moscow's Domodedovo International Airport was bombed with similarly deadly results in January 2011.
St. Petersburg, Russia's second city, a cultural center and President Vladimir Putin's hometown, had escaped unscathed until Monday's bombing. Putin was in the city at the time of the attack.
The city had been previously affected by the downing of a St. Petersburg-bound airliner in October 2015. It had departed the Sinai resort of Sharm el Sheikh and was blown up over the Peninsula, killing all 224 passengers and crew on board. The so-called "Islamic State" (IS) claimed responsibility, citing Russia's military involvement in Syria.
Russia's ambassador to Turkey, Andrei Karlov, was shot dead at an Istanbul art exhibition in December 2016 by a Turkish security officer, also in response to Russia's involvement in Syria. IS has threatened Russia with additional attacks. Russia's internal security service, the FSB, regularly reports arrests of alleged extremists and thwarted attacks.
Ten suspected IS sympathizers were arrested in St. Petersburg and Moscow in November 2016. Russia said it disrupted a New Year's attack as a result. An earlier IS attack was reported to have been averted in Nizhny Novgorod, a city in the Volga region.
In March, IS fighters attacked a Russian security installation in Chechnya. Six soldiers and six attackers were killed.