The first funerals have been held for victims of two suicide bombings that ripped through the southern Russian city of Volgograd less than 24 hours apart.
Twenty-nine-year-old Sergei Nalibayko and 27-year-old Leonid Sushchev were buried in city cemeteries on December 31, with funerals for at least two more victims planned the following day.
Russian media are crediting Nalibayko with having saved some people's lives after noticing the suicide bomber's suspicious behavior at the main railway station and confronting the bomber.
The train-station bombing on December 29 and a blast on a trolleybus on December 30 have left at least 34 people dead and dozens more injured, some of them still in critical condition.
At least four of those wounded in the attacks have been taken to Moscow for medical treatment.
In response to the attacks, Russian authorities have placed some 5,200 police and Interior Ministry troops on the streets and on public transport in Volgograd.
Volgograd Governor Sergei Bozhenov said volunteers were helping the police.
"[We] have Cossack volunteers who, together with police, are patrolling the streets," Bozhenov said. "The number of Cossack volunteers is increasing."
Volgograd police say more than 1,500 homes and other buildings have been searched and that process will continue.
Volgograd police spokesman Andrei Pilipchuk said 152 various firearms and 4.7 kilograms of narcotics have been seized since the police operation started in the wake of the attacks.
Sweeps of the city have also led to the detention of 87 people, most for resisting arrest or possessing invalid documents.
Sochi The Target?
In response to the attacks, the United States has offered "closer cooperation" with Russia on security preparations for the 2014 Winter Olympics, which are due to open in the Black Sea resort of Sochi in less than six weeks.
Volgograd, formerly Stalingrad, is located about 700 kilometers from Sochi.
There has been no claim of responsibility for the bombings, and it remains unclear why Volgograd may have been targeted.
The Emergency Situations Ministry said two more victims succumbed overnight on December 30-31 to injuries from the blasts.
Russia's Foreign Ministry has described the attacks as part of a global Islamist militant campaign that includes the involvement of militants in the North Caucasus and urged international solidarity against terrorism.
In a statement, the ministry compared the Volgograd bombings to attacks in the United States, Syria, Afghanistan, and elsewhere.
It named Doku Umarov, the Chechen warlord who leads the insurgency in the North Caucasus, as among those fomenting violence.
Investigators said evidence suggests that the latest bombings were connected and were likely organized by the same group.
Russian Investigative Committee spokesman Vladimir Markin said the shrapnel used in the two explosive devices was identical.
He said that in the train-station attack, investigators believe the explosives were detonated by a male suicide bomber whose remains have been taken for genetic analysis in a bid to establish his identity.
Canine units with security forces after the December 30 bomb tore through a trolleybus near a downtown market in Volgograd.
Markin said the trolleybus bomb had the force of the equivalent of 4 kilograms of TNT.
President Vladimir Putin has ordered increased security across Russia, with a special emphasis on transportation hubs and crowded venues.
Russian Federal Security Service (FSB) Director and National Antiterrorism Committee Chairman Aleksandr Bortnikov told reporters in Volgograd that extra troops and civic organizations were being deployed to boost safety.
"We are mobilizing additional units of internal troops that will patrol the streets together with regular police. We are also mobilizing civic organizations, namely Cossacks. Civilian volunteers are also being involved to ensure order," Bortnikov said.
"I understand that these measures stem out of necessity, but at the same time we must understand that security is not an exclusive business of the law enforcement structures -- it also concerns society as a whole."
The United Nations said Secretary-General Ban Ki-moon had spoken to Putin to express condolences over the attacks.
The UN said Putin had pledged to the UN chief that Russia will remain within the bounds of international law as it hunts for the perpetrators of the bombings.
In Washington, White House National Security Council spokeswoman Caitlin Hayden said in a statement that the "United States stands in solidarity with the Russian people against terrorism."
The statement added that the Obama administration has offered its "full support to the Russian government in security preparations for the Sochi Olympic Games."
It said the U.S. administration would "welcome the opportunity for closer cooperation" with Moscow to ensure the safety of Olympic athletes, spectators, and other participants.
Russian Olympic officials have pledged that strict security measures are being implemented in Sochi to protect the Olympics, which are due to open on February 7.
The International Olympic Committee said on December 30 that it remains confident Russian authorities would deliver a safe and secure Olympics.
With reporting by Reuters, AP, and ITAR-TASS