Another deadly blast has rocked the Russian city of Volgograd, killing at least 14 people aboard a trolleybus during today's morning commute. The explosion comes a day after a suicide bombing in the city killed at least 17 people and injured more than 40.
The explosions raised concerns about terrorism six weeks before Russia hosts the 2014 Winter Olympic Games in Sochi.
At least 14 people died in the trolleybus explosion and at least 28 people injured, according to the Russian Health Ministry. A 5- to 7-month-old baby suffered multiple head injuries and was unlikely to survive, Health Minister Veronika Skvortsova said. The baby's gender has not been disclosed.
A total of 27 people are in hospitals, including three children, Skvortsova said. The condition of most patients is "from relatively satisfactory to moderately severe," she added.
Russian President Vladimir Putin ordered the National Anti-Terrorist Committee to tighten security measures across Russia, with the additional measures in Volgograd.
The White House condemned the attacks and offered its "deepest condolences to the families of the victims" before saying the U.S. government would "welcome the opportunity for closer cooperation for the safety of the athletes, spectators, and other participants" at the upcoming Sochi Olympics.
U.S. officials have said that Russian cooperation on anti-terror strategy for the Olympics "could be better," according to an analysis written by Homeland Security Policy Institute Director Frank Cilluffo and LAPD Deputy Chief of the Counter-Terrorism Bureau Michael Downing and featured on ABCNews.com today.
The most recent explosion was apparently set off by a male suicide bomber and remnants of his body have been sent for identification, Russian Investigative Committee spokesman Vladimir Markin said.
Investigators say that Monday's blast and the previous day's terrorist attack in a train station are possibly connected.
No group has said it was behind either blast.
The blast happened near one of the city's markets and ripped away much of the bus' exterior and broke windows in nearby buildings. The Investigative Committee said the attack was equivalent to about 8.8 pounds of TNT.
With the Winter Olympics set to open 400 miles away in Sochi, officials are increasingly concerned about the prospect of a terror attack.
"I think the Russian government has something to fear and that is the potential loss of face, the potential embarrassment to them if this terrorist syndicate is able to pull off one or more major terrorist events," said Christopher Swift, professor of National Security Studies at Georgetown University.
The International Olympic Committee expressed its condolences over Sunday's bombing in Volgograd, but said it was confident of Russia's ability to protect the Games.
"This is a despicable attack on innocent people and the entire Olympic Movement joins me in utterly condemning this cowardly act," IOC President Thomas Bach said. "I have personally written to the President of the Russian Federation, Vladimir Putin, to express our condolences to the Russian people and our confidence in the Russian authorities to deliver safe and secure Games in Sochi."
"Sadly terrorism is a global disease but it must never be allowed to triumph," Bach said.
People in Volgograd are terrified, with many refusing to take public transport, according to Russian media reports.
Sunday's suicide attack was carried out by a female Russian, authorities said. Authorities said the attack left 27 people seriously injured, and that the death toll may increase. A police officer was among the dead, and a 9-year-old girl was injured, authorities said.
The train station attacked Sunday is one of the five largest in Russia and is a major transit point for much of southern Russia. It was unclear whether the train station was the woman's intended target or whether she planned to travel elsewhere to carry out an attack.
Surveillance video showed the exact moment the explosion took place, just inside an entranceway to the train station.
There have been 32 terrorist attacks in Russia in the year ahead of Sochi 2014, according to Kavkazskiyuzel, a Russian think tank.
"With these kinds of operations, it's not the size of the operation that matters -- it's the willingness of the people executing the operation to target civilians," professor Swift said.
Chechen rebel leader Doku Umarov called for new attacks earlier this year against civilian targets in Russia, including the Sochi Games. Umarov urged his followers to "do their utmost to derail" the Sochi Olympics, which he described as "satanic dances on the bones of our ancestors."
The Volgograd region has declared a period of mourning until Jan. 3 for the victims of both bombings. The city, formerly called Stalingrad, also serves as an important symbol of Russian pride because of a historic World War II battle in which the Soviets turned the tide against the Nazis.
ABC News' Dragana Jovanovic, Ben Gittleson, Tomek Rolski and The Associated Press contributed to this report.