full-moscow-subway-RTR2C8ZM_

People place flowers at Lubyanka metro station in Moscow on Tuesday as Russia observed an official day of mourning for the 39 victims of the suicide bombings that occurred Monday at Lubyanka and another subway station. ((Alexander Natruskin/Reuters))

Russians were observing an official day of mourning Tuesday after suicide bombers attacked two Moscow subway stations during the rush-hour commute Monday morning, killing 39 people.

No group has claimed responsibility for the blasts at Lubyanka and Park Kultury stations, but officials have suggested militants from the country's restive North Caucasus region were involved.

YOUR REPORT: If you were caught up in the Moscow chaos, send us your stories and photos.

In a televised interview Tuesday, Prime Minister Vladimir Putin said he is confident security officials will find the organizers of the attacks.

"We know that they are lying low, but it is already a matter of the pride of law enforcement agencies to drag them out of the sewer and into broad daylight," Putin said.

Putin's choice of words recalled his famous threat to "wipe out the Chechen rebels in the outhouse" after he blamed them for a series of apartment building bombings that terrorized Moscow in 1999.

Putin, who was prime minister at the time, sent in overwhelming military force to pound the region into submission and was elected president the following year. That has raised fears of a similar crackdown on civil liberties this time around.

The first blast happened around 8 a.m. local time Monday as a train pulled into Lubyanka station in central Moscow. About 45 minutes later, a second blast hit the Park Kultury station on the same subway line, near Gorky Park.

In both cases, investigators believe female suicide bombers detonated bombs they were carrying as the trains pulled into the stations and the doors were opening.

No group has claimed responsibility for the attacks, but speculation is that that they were retaliation for the recent killings of Islamic militant leaders in the North Caucasus, including one known for training suicide bombers.

Andrei Seltsovsky, a city health department official, told the Rossiya-24 state news channel that five people were still in critical condition out of 71 admitted to hospital after the blasts. Only eight victims have been formally identified, he said.

Russians return to metro

tp-moscow-mourns-8398031

People light candles outside Lubyanka station for victims of the Moscow subway explosions. ((Sergey Ponomarev/Associated Press))

The metro system was reopened late Monday afternoon, in time for the evening commute.

Police with machine guns and sniffer dogs were sent to patrol some subway entrances, and heightened security was still in effect across the capital and other Russian cities.

On Tuesday morning, many Russians returned to the metro system, which carries about seven million passengers on an average weekday. Commuters filled the stations with flowers and candles.

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

But Anna Sazonova said she's trying hard not to feel fear as she makes her way to work.

"We should not feel fear because of these attacks," she said "We should feel as usual. Because if we do, then, those people achieve their aim."

Monday's attacks were the first confirmed terrorist attack on Moscow since August 2004, when a suicide bomber blew herself up outside a subway station.

Opposition fears crackdown

russia-security-RTR2C8C4

Interior Ministry officers keep watch at Park Kultury metro station, one of two Moscow subway stations targeted by suicide bombers Monday. ((Denis Sinyakov/Reuters))

Some senior politicians have expressed concerns that civil liberties may be sacrificed under the pretext of fighting terrorism — a charge Putin faced during his eight-year presidency.

"I understand what authorities will do," opposition leader Boris Nemtsov said in an editorial published in an online magazine. "They will resume persecution of opposition, there will be more censorship, political spying.

"There will be more riot police dispersing opposition rallies and protests. But it will not save us from terrorism."

As president, Putin consolidated control in the wake of the 2004 Beslan school hostage crisis by abolishing the election of regional governors.

With files from The Associated Press