Russian police said yesterday they had detained two suspects, including a Chechen, following Tuesday's bombing in Moscow which killed seven people and injured more than 90.
President Putin warned against any witch hunts after the blast but said the war in the rebel region would be pursued to its final goal of destroying guerrillas in their "lair".
The Chechen President, Mr Aslan Maskhadov, denied his separatist forces were involved in the blast, which tore through tunnels leading from a main underground railway station in central Moscow during the rush hour on Tuesday.
The site of the blast has become a place of mourning, with people laying flowers and standing, some in tears, staring at the devastation.
Mr Putin, who took personal control of the investigation into the blast, said it might have been a settling of accounts between criminal gangs, or terrorism. If it was a terrorist act, a single nation should not be blamed, he said.
"It is very wrong when we brand one nation, because criminals, terrorists above all, do not have a nation or a belief," Mr Putin said. His comments, shown in part on Russian television, were reported fully in a Kremlin statement.
"We must bring to a conclusion what we are doing in the North Caucasus: we need to hit the terrorists in their lair, we need to protect people from such acts in other parts of the Russian Federation," he said.
A link to Chechnya, where Russian forces are fighting separatist rebels, was on the mind of most Russians after the blast and brought back fearful memories of a wave of bombings in Russia last year which killed nearly 300 people.
The blasts were one of the reasons Russia invaded Chechnya, starting its second war in the region in three years, although rebels denied any link to the blasts. Mr Putin's own rise was linked to his strong support for the war.
The first suspects in Tuesday's bomb held by the FSB security police were two men, one from Chechnya and the other an Avar from a neighbouring North Caucasus region, Dagestan.
"We do not exclude they were both behind the terrorist act," said Mr Vladimir Pronichev, first deputy chief of the FSB domestic security police. Interfax quoted the FSB as saying three other Chechens had been detained on suspicion of planning a bomb attack in the southern town of Saratov.
As police tightened security throughout the capital, the search for other suspects went on. The Interior Minister, Mr Vladimir Rushailo, appeared on television to show photo-fits of three men. RIA news agency quoted the Moscow police official, Mr Alexander Oboidikhin, as saying a fourth person, believed to be the main culprit, was also being sought.
Mr Rushailo said the death toll was seven, with 93 people receiving medical treatment. The Emergencies Ministry denied a report that an eighth person had died yesterday.
Russian media said witnesses described seeing two men leave a bag or bags near a kiosk to go and change dollars into roubles. They were then seen rushing away before driving off in a Russian-made car just moments before the blast.
Experts said the blast beneath central Tverskaya Street, just a few blocks from the Kremlin, was caused by between 400 grams and 1.5 kg of explosives.
Despite the damage, police were able to reopen the tunnels after an initial clear-up operation. In the light of makeshift lamps, people walked hurriedly down the tunnel which the night before was filled with blackened debris, shattered glass and twisted metal. Many seemed to want to get out of the tunnel as fast as possible although others stopped to lay flowers. A wave of sympathy for the injured caused people to stand in long queues to give blood.