Twin bomb blasts allegedly detonated by female bombers ripped through Moscow's subway system at the height of rush hour this morning, killing at least 38 people, according to Russian emergency officials.
Moscow Mayor Yuri Luzhkov told reporters that "two female suicide bombers were involved" in the attacks, which struck central Metro stations Lubyanka and Park Kultury at 7:56 a.m. and 8:39 a.m. local time.
The fact that the bombers are believed to be women has led many to fear that it may be the work of the notorious "black widows", female suicide bombers from Chechnya who have attacked Moscow before.
Police are expected to release CCTV footage of the suicide bombers and accomplices later today, the Russian news agency Interfax is reporting.
The surveillance video from inside the train showed the bombers "accompanied by two women of Slav appearance. The faces of all of them were exposed," a source told Interfax.
Emergency services say more than 60 people were injured in the blasts that struck at the busiest time of day. Moscow has one of the most used subway systems in the world, with millions travelling on it every day.
Witnesses told Reuters news agency that people struggled to escape from the two stations, as thick smoke flooded the underground.
"I was in the middle of the train when somewhere in the first or second carriage there was a loud blast. I felt the vibrations reverberate through my body," an unidentified man at Park Kultury told the RIA news agency.
"People were yelling like hell," he said. "There was a lot of smoke and in about two minutes everything was covered in smoke."
According to the BBC, ambulances were fighting traffic to get to the injured. News channel Russia Today reported that some of the wounded were being evacuated by helicopters because of the blocked roads.
According to Reuters, surveillance television footage posted online from the Lubyanka station lobby showed several bodies lying on the floor or collapsed against the wall as emergency workers tried to treat the injured.
"I was moving up on the escalator when I heard a loud bang, a blast. A door near the passage way arched, was ripped out and a cloud of dust came down on the escalator," a man named Alexei told the state-run Rossiya 24 television channel.
"People started running, panicking, falling on each other," he said.
Lubyanka station is located near the headquarters of the Russian domestic security service FSB. Park Kultury is located opposite the famous Gorky Park.
Prosecutors told Reuters that they have opened a terrorism investigation after forensic experts found the remains of a female bomber at one of the blast sites.
In an on-camera meeting with President Dmitry Medvedev, the head of the Federal Security Service, Alexander Bortnikov, said a preliminary investigation suggested the involvement of terrorists from Russia's volatile Caucasus region.
"People will look toward the North Caucasus as that is where these things have emanated from in the past," James Nixey a fellow of the London based think tank Chatham House told ABC News.
He said Russian Prime Minister Vladimir Putin, who was returning home early from holiday to spearhead Moscow's response, would likely increase military activity in the region.
"There must be a response for the sustainment of the regime," Nixey said, alluding to Putin's strongman image and previous promises of stamping out the militancy in Chechnya.
Putin led Russia into its second post-Soviet war against rebels in the North Caucasus province of Chechnya after a series of deadly apartment building bombings in Moscow and other cities.
Putin at one point vowed that rebels would be tracked down and killed even "in the outhouse," typical of the tough talk that bolstered his popularity during his 2000-2008 presidency.
"This is a direct affront to Vladimir Putin, whose entire rise to power was built on his pledge to crush the enemies of Russia," Jonathan Eyal of Britain's Royal United Services Institute said of the bombings.
President Obama condemned the attacks, saying the "American people stand united with the people of Russia in opposition to violent extremism and heinous terrorist attacks that demonstrate such disregard for human life."
President Medvedev said the government would act "without compromise" against terrorism. "We shall continue operations against terrorists without wavering and to the end."
In response to the attacks, Russia's civil aviation authorities have ordered all local airports to increase security, a spokesman for the authorities told Reuters.
The NYPD told ABC News today that it " is increasing police coverage of the New York City subway system as a precaution" after the Moscow bombings.
No group has yet claimed responsibility for the attack, which is the worst attack to hit the Russian capital since February 2004, when a suicide bombing on the subway killed at least 39 people.
Many Muscovites fear a return of the Black Widows, the name given to a group of female suicide bombers from Chechnya.
They first appeared during the Moscow theatre hostage crisis in 2002. Dressed from head to toe in black they sought revenge for the loss of their male relatives killed by the Russian military in Chechnya.
Also dubbed the Brides of Allah these female suicide bombers have struck in Moscow before with devastating effect.
In August 2004 a woman detonated herself outside a metro station, killing 10 and in July 2003 two female suicide bombers hit a concert killing at least 15. Also in 2004 female suicide bombers set off explosives on two passenger jets.
In November last year, a bombing on the Nevsky Express north of Moscow, killed 26 people. An Islamist separatist group from the North Caucasus claimed responsibility for that attack.
Both stations hit today are now open. A fast clean up job has removed the blood and debris but the walls are scarred with shrapnel marks.
Passengers entering them were fearful.
"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."
Mark Crudele and Reuters contributed to the reporting of this story.