At least 14 people were killed 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.
The morning rush-hour blast -- the second in less that 24 hours in the city -- left bloodied bodies on the street and raised worries about more violence in the lead-up to the Winter Olympics that Russia will host in February.
"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?"
Russian investigators called the blast a "terrorist act," according to Reuters.
"A criminal case has been launched after the explosion in the trolley bus in Volgograd," officials said in a statement in the blast's aftermath. "Preliminary findings show at least 10 are dead, 15 wounded."
The National Anti-Terrorism Committee, the country's main investigative agency, said the bus blast came from a bomb that most likely had been planted in the vehicle's passenger area, according to The Associated Press.
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.
On Sunday, at least 16 people were killed in a terror attack by a suspected female suicide bomber at a railway station in the city. 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.
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