Six people were killed and 14 were injured in central Moscow yesterday in what the authorities said appeared to be a double suicide bombing that went wrong.
Several of the dead were students from the nearby Moscow University building, just yards from the Kremlin.
It was the second bomb attack in Russia in four days, following Friday's explosion on a commuter train in southern Russia, which claimed 44 lives.
Militants from the breakaway region of Chechnya were immediately suspected. President Vladimir Putin, addressing regional leaders in the Kremlin, blamed unspecified "terrorists", who threatened Russia's development.
The attack took place in mid-morning in front of the National Hotel, a luxury establishment close to Red Square and around the corner from Moscow's main shopping street. There were two explosions, two hours apart, in front of the hotel. The first caused the deaths and injuries. The second was apparently a controlled explosion triggered by a detection robot that found further explosives in a handbag.
There was considerable confusion in the minutes after the first explosion. Bodies were strewn over the street and passers-by and hotel staff were attempting to help. A Reuters correspondent reported seeing the severed head of a woman close to a briefcase in the snow. Witnesses spoke of hearing an explosion, then seeing smoke and smelling burnt flesh.
Four of the hotel's plate glass windows had huge, jagged gashes, and there was some superficial damage to the interior. A Mercedes parked outside the hotel was destroyed and other cars were damaged. Initially, it was thought that the Mercedes had been used as a car bomb, but officials later expressed doubt.
The police and security services did not exclude the possibility that the bombs were linked to organised crime. Another theory was that the intended target of the attack was the state Duma (parliament) building, across the street from the hotel.
Witnesses said that two women had asked passers-by, seconds before the explosion, how to get to the Duma. Officials suggested that the bomb may have exploded prematurely, before the two women could reach their real target.
The Mayor of Moscow, Yuri Luzhkov, newly re-elected with a huge majority, said it was a botched attack by at least one, but probably two, women suicide bombers. "Clearly, the bomb went off by accident," he told the Interfax news agency. "The National Hotel was not the place where the suicide bombers had planned to stage the explosion."
A spokesman for Moscow police, Yevgeni Gildeyev, said investigators were increasingly convinced that the bombing was linked to terrorism rather than organised crime. The Russian security service, the FSB, also favoured the terrorism theory. Russian "mafia" killings have increasingly been commissioned shootings, with high-profile targets, rather than indiscriminate killings.
Officials said that one of the presumed bombers had been killed in the blast. They said the bomb consisted of nails, ball bearings and metal shavings, as well as explosives. They suggested that one or possibly two other people, at least one of them a woman, could have been responsible, and that the main bomber appeared to have had explosives around her waist - a characteristic of the Chechen attacks.
Mr Putin used the attack to underline the need to prevent separatists tearing Russia apart. In a reference to Russia's Constitution Day, which falls on Friday, Mr Putin said the constitution was "a foundation for the development of a free-market economy, democracy, and the development of the nation as a whole and the preservation of its territorial integrity... The actions of criminals, terrorists, which we have to confront even today, are aimed against all that."
After the north Caucasus region, which is adjacent to Chechnya, Moscow has been one of the places most affected by bombings. Three people were killed in a suicide bombing at an outdoor rock concert near Moscow in July, and in October 2002a Moscow theatre audience was held hostage by a group of Chechen militants with explosives strapped to their bodies. The militants were killed when security forces stormed the building, but as many as 100 people died from the gas used by the security forces to disable them.