People stand at the sight of the explosion at Park Kultury subway station in Moscow, Russia, Tuesday, March 30, 2010. Flowers overflowed Tuesday from rickety tables in two Moscow subway stations in memory of the 39 passengers killed in a double suicide bombing as Russia began a national day of mourning.

MOSCOW — Russians today nervously returned to subway stations where two suicide bombers killed 39 people, lighting candles and leaving heaps of carnations at one site as the country began a day of mourning.

The city remained on edge after Monday's attacks, which shocked a country that had grown accustomed to such violence being confined to a restive southern corner — and marked the return of terrorism to the everyday lives of Muscovites after a six-year break.

Many have speculated that the blasts, blamed on North Caucasus rebels, were retaliation for the recent killing of separatist leaders in the region by Russian police.

"I feel the tension on the metro, nobody's smiling or laughing," said university student Alina Tsaritova, not far from the Lubyanka station, one of the targets.

The preliminary investigation found that female suicide bombers detonated belts of explosives during the Monday morning rush-hour at the stations.

Five people remain in critical condition out of 71 hospitalized after the blasts, city health department official Andrei Seltsovsky told the Rossiya-24 state news channel. Only eight victims had been formally identified, he said.

Some commuters said today they would try and block the events out of their mind completely."We have to live with this, not to think about it, especially when we're underground," said Tatyana Yerofeyeva, a Muscovite in her early 50s.Plastic plaques hung in the two metro stations above rickety tables overflowing with flowers; their inscriptions promised permanent replacements. Some people were choked by tears as they laid candles.Flags flew at half staff on government buildings, at the Kremlin, and in other cities across the vast country. Entertainment events and television shows were canceled, and services were scheduled at several churches.Heightened transportation security remained in effect across the capital and elsewhere. Police with machine guns and sniffer dogs patrolled subway entrances.Monday's first explosion took place just before 8 a.m. at the Lubyanka station in central Moscow, beneath the notorious headquarters of the Federal Security Service or FSB, the KGB's main successor agency.About 45 minutes later, a second blast hit the Park Kultury station.

A religious service is conducted today at Christ the Saviour Cathedral in Moscow for the victims of Monday's subway bombings. Authorities say 39 people were killed in the double suicide blasts.