By Albina Kovalyova and Daniel Arkin
MOSCOW -- At least 14 people were killed and 28 wounded when an explosion ripped through a trolley bus in the Russian city of Volgograd on Monday, the country's third deadly attack in four days.
Members of the emergency services work at the site of a bomb blast on a trolleybus in Volgograd December 30, 2013. (Reuters / Sergei Karpov)
The explosion, which authorities blamed on a suicide bomber, tore out much of the electric bus' exterior, left mangled bodies on the street and raised fears about more violence in the lead-up to the Sochi Winter Olympics that Russia will host in six weeks.
"For the second day, we are dying -- it's a nightmare," a woman near the scene told Reuters, her voice trembling. "What are we supposed to do, just walk now?"
Volgograd, a city of around one million about 400 miles northeast of Sochi, is a key transport hub for southern Russia, with many bus routes linking it to the volatile provinces in the North Caucasus.
On Sunday, 17 people were killed in a terror attack by a suspected female suicide bomber at a railway station in Volgograd. And on Friday, a car bomb killed three people in the southern Russian city of Pyatigorsk, 170 miles east of Sochi, where the Olympics will be held.
In October, another female suicide bomber was blamed for a bus explosion in Volgograd that killed five people.
The blasts on Sunday and Monday were probably connected because the TNT and shrapnel-packed bombs were "identical," said Vladimir Markin, spokesman for Russia's main investigative agency.
"That confirms the investigators' version that both terrorist attacks were linked," he told Russian television. "They could have been prepared in one place."
The remains of the presumed bomber had been collected and were being examined, Markin added.
Earlier, federal investigators had said that the bus blast came from a bomb that most likely had been planted in the vehicle's passenger area, according to The Associated Press.
There was no immediate claim of responsibility for the attacks, but the leaders of an insurgency that aims to create an Islamic state out of Muslim provinces south of Volgograd have urged militants to use "maximum force" to stop the Olympics from being held.
President Vladimir Putin called a meeting of security and intelligence chiefs in the wake of Monday's attack, and the country's interior ministry ordered police to bolster patrols in railway stations and other transport hubs across the country.
Putin, who was elected after waging war against Chechen rebels in the mountains just south of Volgograd, has staked his reputation on organizing a safe Olympics. Security experts warn that his biggest security challenge is attacks by Islamist militants whose fight is rooted in this war.
“Since these games were first awarded to Russia several years back, people were worried because of the long-standing conflicts,” NBC News counter-terrorism analyst Michael Leiter said after Sunday’s attack. “And this type of mass transit is what officials are most concerned with.”
Russia on Monday compared the twin bombings to attacks by militants in the U.S., Syria and other countries around the world, and called for international solidarity in the fight against "terrorists."
"We will not retreat and will continue our consistent fight against an insidious enemy that can only be defeated together," the foreign ministry said in a statement. It added that the attacks came against the backdrop of threats from militants such as Doku Umarov, an Islamist leader who has called on fighters to sabotage the Olympics.
The White House issued a statement within hours of the latest blast, saying President Barack Obama has been briefed on the attacks and the United States and Russia were cooperating on anti-terrorism leading up to the Games.
Russian Olympic Committee President Alexander Zhukov said Monday that there was no need to take any extra steps to secure Sochi in the wake of the attacks because "everything necessary has been done."
NBC News' F. Brinley Bruton, The Associated Press and Reuters contributed to this report. Daniel Arkin reported from New York.