The suicide bombing on a trolleybus in the southern Russian city of Volgograd came a day after 17 people died in another suicide attack at the central station in the city.
With the Winter Olympics in Sochi, about 400 miles from Volgograd, due to open on 7 February, the British Olympic Association (BOA) described the terror attacks as “a painful reminder of the threats that exist in our world today”.
The authorities said they believe it was the work of the same group which set off a bomb at the railway station a day earlier.
Together more than 30 people were killed in the explosions, putting the city of one million on edge and highlighting the terrorist threat Russia is facing as it prepares to host February’s Winter Games in Sochi, President Vladimir Putin’s pet project.
While terrorists may find it hard to get to the tightly guarded Olympic facilities, the bombings have shown they are able to hit civilian targets.
“As with every Olympic Games – winter and summer – it is the responsibility of security officials at the national level, working in close co-ordination with regional and local authorities and the Games Organising Committee, to ensure that the environment is as safe and secure as possible,” a BOA spokesman said.
“Throughout their planning for the 2014 Olympic Winter Games, Russian officials have indicated that security would be among the highest priorities, and everything we have seen in their planning would indicate this is the case.
“For the British Olympic Association, we have no higher priority than the safety and security of our delegation.”
Volgograd serves as a key transport hub for southern Russia, with numerous bus routes linking it to volatile provinces in Russia’s North Caucasus, where insurgents have been seeking an Islamic state.
Vladimir Markin, the spokesman for Russia’s main investigative agency, said yesterday’s explosion involved a bomb similar to the one used in Sunday’s attack at the city’s main railway station. He said: “That confirms the investigators’ version that the two terror attacks were linked. They could have been prepared in one place.”
Mr Markin said a suicide attacker was responsible for the bus explosion, reversing an earlier official statement that the blast was caused by a bomb left in the vehicle’s passenger area.
At least 14 people were killed and nearly 30 were wounded, according to public health officials.
No-one has claimed responsibility for either bombing, but they came several months after Chechen rebel leader Doku Umarov threatened new attacks against civilian targets in Russia, including the Olympics in Sochi. Suicide bombings and other terror attacks have rocked Russia for years, but most recently have been confined to the North Caucasus region.
Yesterday’s explosion ripped away much of the bus’s exterior and shattered windows in nearby buildings. It virtually paralysed public transport in the city, forcing many residents to walk long distances to get to work.
Russian authorities have been slow to introduce stringent security checks on bus routes, making them the transport of choice for terrorists in the region.
The Interior Ministry ordered police to beef up patrols at railway stations and other transport facilities across Russia. Mr Putin on Monday summoned the chief of the main KGB successor agency and the interior minister to discuss the situation, and sent the former to Volgograd to oversee the probe.
Umarov ordered a halt to attacks on civilian targets during the mass street protests against Mr Putin in the winter of 2011-12. He reversed that order in July, urging his men to “do their utmost to derail” the Sochi Olympics, which he described as “satanic dances on the bones of our ancestors”.
Anyone wanting to attend the Games will have to buy a ticket online from the organisers and obtain a “spectator pass”. Doing so will require providing passport details and contacts that will allow the authorities to screen all visitors and check their identities upon arrival.
The security zone created around Sochi stretches approximately 60 miles along the Black Sea coast and up to 25 miles inland.