It has been several years since the Russian capital was last hit by a major terrorist attack, but yesterday's events recall the early years of Vladimir Putin's presidency, when Chechen terrorists struck at trains, planes and even a theatre in the city. They suggest that a decade after Mr Putin's promise to track down terrorists and "waste them in the outhouse", the Caucasus insurgency still has the ability to strike at the heart of Russia.
Few doubt that the atrocities are the work of groups operating in Chechnya and the other republics of the North Caucasus. The apparent use of female suicide bombers, the "Black Widows" employed in many previous attacks by Chechen rebels, also suggests a link to the restive region.
Russia has fought two wars in Chechnya, and in recent years has trumpeted the peace and stability brought to the province under the Kremlin-backed hardman Ramzan Kadyrov. Despite allegations of brutality and torture made against Mr Kadyrov's forces, the situation in the republic is ostensibly fairly stable, with only infrequent terrorist attacks. In neighbouring Dagestan and Ingushetia, however, attacks on police and local officials occur on a daily basis. Analysts put the violence down to a potent mix of Islamic fundamentalism, local separatism, a reaction against the brutality of authorities across the region, and endemic corruption.
Despite the rumbling unrest across Russia's south, the authorities have succeeded in keeping terror away from Russia's major cities. But in recent months, the separatists have announced several times that they planned to hit the Russian heartland. The Chechen rebel leader Doku Umarov claimed responsibility for a bomb that derailed the Nevsky Express train travelling from Moscow to St Petersburg last November, and in February warned that "the war is coming to their cities".
Russians will hope that yesterday's bombings are an isolated incident, but it is difficult to judge just how much capacity the rebels have to launch more attacks. Mr Kadyrov has claimed on many occasions that the insurgency has been wiped out and that only a few "bandits" remain in the mountains. However, analysts say that while the Chechen fighters have indeed been severely weakened, the brutal policies of Mr Kadyrov, coupled with high levels of corruption and poverty in the North Caucasus, make fertile ground for attracting new recruits.
Last week, a top Russian army official estimated that there are around 500 separatist fighters operating in the North Caucasus. The fighters are constantly on the move, hiding out in the mountainous terrain of Chechnya and its neighbouring republics, or finding shelter with sympathetic families. In recent years, their statements have shifted from localised, separatist concerns to more overtly Islamist, jihadi rhetoric.
In 2006, Russian security forces announced the death of Shamil Basayev, Russia's answer to Osama bin Laden, depriving the resistance of its most charismatic figurehead. Basayev claimed to have organised a number of atrocities, the most horrific of which was the attack on a school in Beslan in 2003, when over 300 died, many of them children.
Since Basayev's death, the resistance has been led by Doku Umarov, who calls himself the Emir of the self-proclaimed Caucasus Emirate. He has so far evaded Russian forces, but this month Said Buryatsky, said to be Umarov's chief ideologue, was killed in a shootout. It is possible that yesterday's attacks were conceived, in part, as revenge.
On some internet forums, opposition-minded Russians speculated that the terrorist attacks may have been supported by elements within the Russian security services, although they offered no evidence to support this theory. Suspicions have long lingered about the series of apartment bombings in 1999 that led to Mr Putin launching the Second Chechen War, with many incongruous details about the blasts lending force to the conspiracy theories.
The political analyst Liliya Shevtsova told Radio Liberty that whoever was behind the attacks, there was likely to be a toughening of the political line from the authorities, as happened after the apartment bombings and the Beslan school siege. Shortly after Beslan, Mr Putin scrapped elections for regional governors and decreed that they would be appointed from Moscow.
"In 2012 there are presidential elections," said Ms Shevtsova. "I can't exclude that in the Russian authorities there are forces who would like to go down the usual route in this situation. The same scheme will work: Terrorist atrocity; threat to national security; strengthening of the regime."