Russian police yesterday arrested two suspects in connection with the explosion in a Moscow apartment building which killed at least 90 people the previous day, Interfax news agency reported.
The agency quoted police sources as saying it was "not excluded" that those arrested had "a link with businesses occupying the ground floor of the destroyed building".
Monday has been declared a day of mourning in Moscow for the victims of three recent explosions in the capital and in Dagestan.
With the number of bodies recovered in Moscow reaching 90, the total for the three blasts is more than 150 deaths.
There was little sign of increased security in the city yesterday, but a nationwide alert has been announced and additional protective measures have been put in place at Russia's nuclear power stations.
The Prime Minister, Mr Vladimir Putin, in a rare television appearance, told viewers it was still not clear why an explosion demolished two sections of a vast apartment building owned by the Moskvich automobile plant at Pechatniki in Moscow shortly after midnight on Wednesday night. If a terrorist act, he said, it was committed by "a particularly sly, malicious, perfidious and bloodthirsty opponent". He added, however, that it could also be a case of "criminal negligence" on the part of people who had illegally stocked the explosives in a residential building.
Moscow's mayor, Mr Yuri Luzhkov, on the other hand, was adamant the explosion was the work of terrorists. "That the deaths were due to a bomb was no longer a hypothesis but an established fact," the mayor said.
Traces of explosive substances were, he said, found in an unoccupied apartment which had been rented by a company called Delko-2. The owners of the company were being questioned.
The Moscow prosecutor's office, however, said it was investigating the possibility that explosives being stored in the office went off accidentally. There were reports on the independent NTV station last night that looting had taken place at the site of the explosion and that a cache of $30,000 had been discovered at the scene, which is in one of Moscow's poorest areas.
Two Moscow newspapers carried a photo-fit picture of a young man wanted for questioning in connection with the explosion.
Last evening, after the body of the 90th victim had been discovered, Mr Anatoly Nikitin, of the Moscow emergency service, said there was little hope of finding any further remains. A bomb in a downtown shopping centre near the Kremlin claimed the life of a young woman on August 31st and 62 died in the bombing of a military apartment block in the Dagestani town of Buinaksk on September 4th.
All three explosions have been linked to the conflict which continues in Dagestan, a mainly Muslim region that borders the Caspian sea. On August 7th the area was invaded by rebels based in Chechnya, some of them members of the Wahhabi sect of Islam, who declared Dagestan to be an independent Islamic state.
As part of their military operations following the bombing in Buinaksk, Russian aircraft bombed villages inside Chechnya and an anonymous caller to the Interfax news agency in Moscow claimed the Pechatniki bombing was carried out in retaliation.
Russian forces claimed yesterday to have captured the village of Gamiyak in their first military victory over the rebels, who are led by the Chechen warlord Shamil Basayev and Jordanian-born Khattab.
Demoralised Russian troops have complained they lack arms to wage a more effective struggle against the rebels.
Moscow has briefed the FBI on the suspected involvement of Islamic radical Osama bin Laden in Dagestan. The Saudi-born millionaire is the FBI's chief suspect for last year's bombing of two US embassies in Africa.