Two women suicide bombers killed at least 38 people and wounded dozens more in rush-hour attacks on Moscow's subway today.
The first 23 victims died in an explosion on a train shortly before 8am at the Lubyanka station underneath the headquarters of the Federal Security Service (FSB), the KGB's successor.
The second explosion hit the Park Kultury station about 45 minutes later killing at least 12 people.
Around 60 more were injured in the blasts thought to have been organised by Chechen rebels.
The attacks came six years after Caucasus Islamic separatists carried out a pair of deadly Moscow subway strikes and raised concerns that the war had once again come to Russia's capital, amid militants' warnings of a renewed determination to push their fight.
Chechen rebels claimed responsibility for a deadly bombing last year on a passenger train from Moscow to St. Petersburg.
Last month Chechen rebel leader Doku Umarov warned that "the zone of military operations will be extended to the territory of Russia ... the war is coming to their cities."
The Moscow subway is one of the world's busiest, carrying around seven million passengers on an average workday, and is key in running the traffic-choked city.
The last confirmed terrorist attack in Moscow was in August 2004, when a suicide bomber blew herself up outside a city subway station, killing 10 people. Responsibility for that blast was claimed by Chechen rebels.
Russian police have killed several Islamic militant leaders in the North Caucasus recently, including one last week in the Kabardino-Balkariya region. The killing of Anzor Astemirov was mourned by contributors to two al-Qa'ida-linked websites.
The militants receive moral and possibly financial support from al-Qa'ida. Dozens of contributors to three websites affiliated with al-Qa'ida wrote in praise of today's attacks.
One site opened a special page to "receive congratulations" for the Chechen rebels who "started the dark tunnel attacks in the apostate countries," and all wished for God to accept the two sisters as martyrs.
"Don't forget Russia's crimes of genocide in the Caucasus and Chechnya," said one writer. "The battle has been shifted to the heart of Moscow," another wrote.
In a televised meeting with President Dmitry Medvedev, Federal Security Service head Alexander Bortnikov said body fragments of the two bombers pointed to a Caucasus connection.
"We will continue the fight against terrorism unswervingly and to the end," Mr Medvedev said.
Prime Minister Vladimir Putin, who built much of his political capital by directing a fierce war with Chechen separatists a decade ago, vowed today that "terrorists will be destroyed."
Although the Russian army battered Chechen rebels in massive assaults a decade ago, the separatists continue to move through the region's mountains and forests with comparative ease and launch frequent small attacks.
The Moscow blasts practically paralysed movement in the city centre as emergency vehicles sped to the stations.
In the Park Kultury blast, the bomber was wearing a belt packed with plastic explosive and set it off as the train's doors opened. A woman who sells newspapers outside the Lubyanka station, Ludmila Famokatova, said there appeared to be no panic, but that many of the people who streamed out were distraught.
"One man was weeping, crossing himself, saying 'thank God I survived'," she said.
By late afternoon the two subway stations reopened and dozens boarded trains.
"It's really terrifying," said Vasily Vlastinin, 16. "It's become dangerous to ride the metro, but I'll keep taking the metro. You have to get to school, to college, to work somehow."
Both stations had been scrubbed clean. Holes left by shrapnel in the granite were the only reminder of the bombings.