At least 14 people were killed Monday when a suicide bomber blew himself up on a packed trolleybus in Volgograd, raising new concerns about security at the Sochi Olympics a day after a deadly attack on the southern Russian city's train station.
President Vladimir Putin, under pressure to show that Russia can assure the safety of tens of thousands of guests when the Winter Games open on February 7, ordered stepped-up security across the country.
The twin suicide attacks on Volgograd, which until this year had no record of recent unrest, have stunned Russia and troubled the authorities as people prepare for mass New Year celebrations.
At least 17 people died in Sunday's attack blamed on a suspected female suicide bomber.
The force of Monday's blast destroyed the number 15A trolleybus, which was packed with early morning commuters and was turned into a tangle of wreckage with only its roof and front remaining.
Health ministry spokesman Oleg Salagai told Russian state television that 14 people were killed and 28 wounded.
Russian investigators have opened a criminal probe into a suspected act of terror as well as the illegal carrying of weapons, the Investigative Committee said.
"The explosives were detonated by a male suicide bomber, fragments of whose body have been found and taken for genetic analysis to establish his identity," said spokesman Vladimir Markin.
He said four kilogrammes (nine pounds) of TNT equivalent had been used and noted that the explosives were identical to those used in Sunday's train station bombing.
"This confirms the theory that the two attacks are linked. It is possible that they were prepared in the same place," he added.
Olympic security fears
The new attack will further heighten fears about security at the Winter Olympics in the Black Sea resort of Sochi, which lies 690 kilometres (425 miles) southwest of Volgograd.
Putin ordered security stepped up across Russia, with a special regime to be imposed in Volgograd, the national anti-terror committee announced.
Federal Security Service (FSB) chief Alexander Bortnikov flew to Volgograd and asked citizens to be understanding about the extra security that may involve spot checks.
"It is a neccessary measure," he said.
Russia is already preparing to impose a "limited access" security cordon around Sochi from January 7 which will check all traffic and ban all non-resident cars from a wide area around the city.
State television said that after the latest blast, Volgograd commuters were abandoning buses and trolleybuses and going to work on foot in fear of a new attack.
The search for the perpetrators is expected to focus on Russia's largely Muslim North Caucasus region where Islamist militants have for years been fighting the Russian security forces.
Doku Umarov, the leader of militants seeking to impose an Islamist state throughout Russia's North Caucasus, has ordered rebels to target civilians outside the region and disrupt the Games.
"This looks like the North Caucasus underground. They promised to stage acts in big Russian cities ahead of the Olympics. It seems they are fulfilling their promise," military affairs commentator Pavel Felgenhauer told AFP.
Hitting the Russian heartland
Moscow city hall's security chief Alexei Mayarov said security would be stepped up in the capital ahead of New Year, the biggest holiday of the year in Russia and traditionally marked by mass outdoor gatherings.
Reports said Russia's second city of Saint Petersburg had already cancelled its planned New Year fireworks display.
Militant strikes have become part of daily life in the North Caucasus. But the Volgograd blasts will be a particular concern to the authorities as the bomber struck a city of more than one million people in the Russian heartland.
The city, known as Stalingrad in the Soviet era, is of huge importance to Russians as the scene of one of the key battles of World War II that led to the defeat of invading Nazi forces.
The city was already attacked on October 21 by a female suicide bomber with links to Islamists who killed six people on a crowded bus.
Russia's double Olympic pole vault champion Yelena Isinbayeva, Volgograd's best known current inhabitant, told ITAR-TASS she felt "terrible, simply terrible" after the attacks.
The blasts are the deadliest in Russia since a suicide bombing at Moscow's Domodedovo airport killed 37 people in January 2011.
Investigators said the suspected female suicide bomber who caused Sunday's bombing set off her charge after being stopped by a police officer at the metal detectors of the central entrance to the station.
Unconfirmed reports identified the bomber as a Dagestani woman named Oksana Aslanova, a so-called "Black Widow" who had been married to two Islamists killed in battles with federal forces. Other reports however said the bomber could have been male.