Russia's Prime Minister Vladimir Putin on Tuesday ordered security forces to snare the masterminds of the metro bombings that killed 39, saying they should be "scraped out" from the bottom of sewers.
The strongman premier's comments came as Moscow marked a day of mourning for Monday's double blasts carried out by female suicide bombers at two busy metro stations at the height of the early morning rush.
Grieving Muscovites added to heaps of flowers and placed photographs of the dead under memorial plaques at the stations. Flags at government buildings flew at half mast and television channels cancelled entertainment programmes.
"We know that they (the organisers) are lying low," Putin said in comments broadcast on state television.
"But it is now a matter of honour for the security forces to scrape them out from the bottom of the sewers and bring them out into the light of day.
"This will be done," he added.
Putin's language was strikingly reminiscent of his famous promise in 1999 to strike at rebels even in the "outhouse" which heralded the adoption of tougher tactics by the authorities against Chechen militants.
Authorities have linked the attacks to militant groups in Russia's largely Muslim North Caucasus, while national security chief Nikolai Patrushev said investigators were looking into a possible link to Georgia.
"We had information that certain members of the Georgian special services were in contact with terrorist organisations in the Russian North Caucasus," he told Interfax news agency.
"We must look into this also when it comes to the attacks in Moscow."
Russia and Georgia fought a brief war in 2008 and the two sides have frequently exchanged accusations.
Russian President Dmitry Medvedev urged steps to make law-enforcement agencies work more efficiently and increase the safety of transport systems and public places.
"We need to focus our attention on certain aspects of improving legislation aimed at preventing terrorist acts," a grim-faced Medvedev said in televised remarks.
Influential daily newspaper Vedomosti had earlier sharply criticised the authorities for failing to prevent the bombings, Moscow's worst attacks in six years.
"In recent years, the authorities and state television have been singing a lullaby to Russians with the thought that terrorism is localised in the North Caucasus and does not threaten ordinary people," it wrote.
Ramzan Kadyrov, the Kremlin-backed leader of the troubled North Caucasus region of Chechnya, joined the call for a crackdown on extremists, saying "terrorists... must be poisoned like rats".
With the atmosphere in Moscow tense, 100 people were evacuated from the gigantic Christ the Saviour Cathedral in the late afternoon after a telephoned bomb threat, police said. A search uncovered nothing.
Police were searching for two women seen with the bombers as well as a possible male accomplice, after identifying them and the bombers through surveillance footage, Russian media reported, citing security sources.
Russian authorities released grainy but grisly photographs to local media showing the severed heads of the two bombers' corpses, which are now the prime evidence in the investigation.
Along with the severed heads, a picture of the man walking to the metro station was also released.
Officials said the death toll rose to 39 -- not including the two bombers -- after a woman died in hospital overnight. The emergency situations ministry said 83 people had been wounded.
Western leaders condemned the attacks and sent messages of solidarity to Russia, which has often been criticised in the West for using brutal counter-insurgency tactics in the North Caucasus.
US President Barack Obama called Medvedev and pledged Washington would "help bring to justice those who undertook this attack," while Pope Benedict XVI condemned the "brutal acts of violence."
There was no immediate claim of responsibility, but the "Caucasus Emirate" group led by Chechen Islamist chief Doku Umarov, said to be behind a November train bombing that killed 28 people, had recently threatened to attack Moscow.
Chechnya, a predominantly Muslim region of the North Caucasus that was the site of two bloody separatist wars after the 1991 collapse of the Soviet Union, has seen rising violence in recent months.