A bomb ripped apart a bus in Volgograd on Monday, killing 14 people in the second deadly attack blamed on suicide bombers in the southern Russian city in 24 hours and raising fears of Islamist attacks on the Winter Olympics in Sochi.
President Vladimir Putin, who has staked his prestige on February's Sochi Games and dismissed threats from Chechen and other Islamist militants in the nearby North Caucasus, ordered tighter security nationwide after the morning rush-hour blast.
Investigators said they believed a male suicide bomber set off the blast, a day after a similar attack killed at least 17 in the main rail station of a city that serves as a gateway to the southern wedge of Russian territory bounded by the Black and Caspian seas and the Caucasus mountains.
A Reuters journalist saw the blue and white trolleybus (a bus powered by overhead electric cables) reduced to a twisted, gutted carcass, its roof blown off and bodies and debris strewn across the street. Windows in nearby apartments were blown out by the explosion, which investigators called a terrorist act.
"For the second day, we are dying. It's a nightmare," a woman near the scene said, her voice trembling as she choked back tears. "What are we supposed to do, just walk now?"
The bomb used was packed with "identical" shrapnel to that in the rail station, indicating they may have been made in the same place and supporting suspicions the bombings were linked, said Vladimir Markin, a spokesman for the investigators.
Health Ministry spokesman Oleg Salagai said 14 people were killed and 28 wounded in the bombing on Monday.
"There was smoke and people were lying in the street," said Olga, who works nearby. "The driver was thrown a long way. She was alive and moaning. ... Her hands and clothes were bloody."
There was no immediate claim of responsibility.
On Sunday, investigators initially described the station bomber as a woman from Dagestan, a hub of Islamist militancy on the Caspian, but they later said the attacker may have been a man. In October, a woman from the North Caucasus blew up and killed seven people on a bus in Volgograd.
The city has held a place in Russians' sense of national identity since, when known as Stalingrad, its Soviet defenders held off German invaders to turn the course of World War Two.
Chechens and other North Caucasus militants have also staged attacks in Moscow and other cities in the past.
Putin, who has not spoken publicly since the attacks, ordered a federal committee that coordinates counterterrorism efforts to step up security nationwide including in Volgograd, and to report to him daily, the Kremlin said.
The violence raises fears of a concerted campaign before the Olympics, which start on February 7 around Sochi, a resort on the Black Sea, 700 km (450 miles) southwest of Volgograd.
In an online video posted in July, the Chechen leader of insurgents who want to carve an Islamic state out of the swathe of mainly Muslim provinces south of Volgograd, urged militants to use "maximum force" to prevent the Games from going ahead.
"Terrorists in Volgograd aim to terrorize others around the world, making them stay away from the Sochi Olympics," said Dmitry Trenin, an analyst with the Moscow Carnegie Centre.
The International Olympic Committee expressed condolences to those affected by the attacks and said "we have no doubt that the Russian authorities will be up to the task" of providing 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," a spokeswoman for the International Olympic Committee in Lausanne, Switzerland.
In power since 2000, Putin secured the Games for Russia and has staked his reputation on a safe and successful Olympics, even freeing jailed opponents including oil tycoon Mikhail Khodorkovsky and the Pussy Riot punk band to remove a cause for international criticism at the event.
Putin was first elected after winning popularity for a war against Chechen rebels, but attacks by Islamist militants whose insurgency is rooted in that war have clouded his 14 years in power and now confront him with his biggest security challenge.
Police said additional officers were being deployed to railway stations and airports nationwide after the bombing at the Volgograd rail station on Sunday, but the attacks raised questions about the effectiveness of security measures.
The police force in Volgograd, a city of a million people on the west bank of the river Volga, has been depleted as some 600 officers were redeployed to Sochi to tighten security around Olympic sites, a police officer told Reuters.
More attacks can be expected before the Olympics and cities in southern Russia where the Games are not being held are easier targets than Sochi, said Alexei Filatov, a prominent former member of Russia's elite anti-terrorism force, Alfa.
"The threat is greatest now because it is when terrorists can make the biggest impression," he said. "The security measures were beefed up long ago around Sochi, so terrorists will strike instead in these nearby cities like Volgograd."
The attacks also threatened to fuel ethnic tension, which has increased with an influx of migrant laborers from the impoverished Caucasus and Muslim Central Asian nations to cities around Russia, including Volgograd, in recent years.
"They need to be chased out of here. It has become a transit junction - there are all these non-Russians, both good and bad," said Olga, a saleswoman at a store near the mangled bus. "We've plenty bandits of our own. Why do we need others?"
Police were checking documents of people in Volgograd, with a focus on migrants, said Russian news agency Itar-Tass.
Volgograd will be one of the venues for the 2018 soccer World Cup, another high-profile sports event Putin has helped Russia win the right to stage, and which will bring thousands of foreign fans to cities around Russia.
The first Olympics in Russia since the 1980 summer Games in Moscow, Sochi is a chance for Putin to show how the country has changed since the collapse of the Soviet Union in 1991.
He has faced criticism in the West and from Russian activists who say he has smothered dissent and encouraged discrimination against homosexuals since starting a third term as president in 2012.
Sunday's attack was the deadliest to strike the ethnic Russian heartlands since January 2011, when a male suicide bomber from the North Caucasus killed 37 people in the arrivals hall of a busy Moscow airport.