VOLGOGRAD, Russia -- Russian President Vladimir Putin has ordered stepped-up security measures across the country after more than 30 people were killed in two suspected suicide bombings in as many days in the southern city of Volgograd.
The Interior Ministry said that vehicles checks will be increased and that "special attention" will be paid to crowded places and transport hubs.
At least 15 people were killed and more than 20 others injured in a rush-hour explosion aboard a trolleybus in the southern city of Volgograd early on December 30.
That blast came less than 24 hours after 18 people were killed and dozens more hurt by an explosion at Volgograd's main train station that officials have described as a terrorist attack by a suicide bomber.
Russian officials put the total number of dead and wounded from both attacks at 104.
A spokesman for Russia's Investigative Committee, Vladimir Markin, said in a statement that the type of explosive used showed the attacks could have been jointly planned.
Markin said the attack was believed to have been carried out by a male suicide bomber. He said fragments of his body have been taken for genetic analysis to establish his identity.
Some Russian media
identify that suspect as Pavel Pechyonkin, whose parents reportedly appealed unsuccessfully in September
for him to "come home" to the Mari El Republic.
WATCH: YouTube video that emerged soon after the December 30 blast, the second deadly explosion in less than 24 hours in Volgograd, a regional administrative hub with 1 million inhabitants:
The explosion occurred near a street market in Volgograd's Dzerzhinsky district.
The trolleybus roof and sides were blown off, and bodies and debris were strewn around the street by the blast.
A spokesman said that "the head of the operations department of the National Antiterrorism Committee has dispatched resources and forces from the Federal Security Service, the Interior Ministry, and the Ministry for Emergency Situations to the scene of the blast.
There has been no claim of responsibility for either attack. Russia's Foreign Ministry said Islamist "bandits" were behind the Volgograd bombings.
The ministry said in a statement on December 30 that the bombers had ties to Chechen Doku Umarov, the leader of the insurgency in the North Caucasus region, who continues to send new fighters into a "war of terror" conducted "under the flag of jihad."
In July, Umarov called on militants to carry out attacks on civilian targets, including strikes to "not permit" the Sochi Olympics
The two explosions come just six weeks before Russia is to host the 2014 Winter Olympics in the Black Sea resort city of Sochi.
The head of the Russian Olympic Committee, Aleksandr Zhukov, told reporters that the authorities had already taken extensive measures to provide security during the games, which open on February 7.
"As for the Olympic Games in Sochi, all the necessary security measures have been taken," Zhukov said. "No additional security measures will be taken in connection with the terrorist acts in Volgograd because everything that is necessary has already been done."
Zhukov said all visitors attending the Olympics will have to obtain special "spectator passes" for access.
He said obtaining such passes requires providing passport details and contacts that will enable authorities to screen all visitors and check their identities upon arrival.
The International Olympic Committee (IOC) in Lausanne, Switzerland, conveyed condolences to those affected by the latest attack and expressed confidence in the ability of Russian authorities to maintain security at the games.
"Unfortunately, terrorism is a global phenomenon and no region is exempt, which is why security at the games is a top priority for the IOC," the IOC said. "At the Olympics, security is the responsibility of the local authorities, and we have no doubt that the Russian authorities will be up to the task."
Volgograd, formerly known as Stalingrad in honor of Soviet dictator Josef Stalin, is a city of around 1 million people that lies 700 kilometers northeast of Sochi. It is also slated to be one of the venues for the 2018 soccer World Cup.
Volgograd has now seen three explosions in the past three months. In October, seven people were killed in an explosion detonated by a female suicide bomber who investigators say had links to Islamists in the North Caucasus.
The Kremlin said Putin had met with Federal Security Service (FSB) head Aleksandr Bortnikov, who informed him about the trolleybus explosion.
Following the December 29 blast, officials announced that security would be tightened at stations, airports, and other facilities.
Officials initially said the suspected bomber of the train station was believed to be a woman, but later suggested a man could have been responsible.
The United Nations Security Council said it condemned the train-station attack "in the strongest terms." Condemnations of the blast were also issued by the U.S. State Department, NATO, and the European Union.
The train-station blast marked the deadliest attack in Russia since January 2011, when a male suicide bomber from the North Caucasus killed 37 people at Moscow's Domodedovo Airport.
With reporting by ITAR-TASS, Interfax, RT.com, AP, and RFE/RL's Russian Service