The death toll in the two attacks -- one Monday on a trolleybus and one Sunday in a railway station -- rose on Tuesday as two of the more than 100 injured died while hospitalized, RIA Novosti reported. The death toll from the trolleybus attack during morning rush hour rose to 16 while the bombing at the train station killed 18.
Russian security forces have begun focusing on Dagestan, a hub of Muslim separatist violence, and the insurgency's leader, Doku Umarov, the New York Times reported.
Umarov had been out of the spotlight until he surfaced in July in a video in which he ordered followers to do whatever possible to attack Russia as it prepared to host the 2014 Winter Olympics in Sochi in February. Largely ignored at the time, his threat now is under review, particularly since he cited Russia's transportation networks as potential targets, experts said.
"The big question is will there be this sort of wave," Gordon M. Hahn, a senior associate with the Center of Strategic and International Studies, told the Times. "This is already a pretty high level -- the fact that they pulled off three suicide bombings in Volgograd in two months. If their idea is to build up a crescendo, they have to take it easy because they'll have to do something really big."
Umarov has claimed responsibility for some of the most horrific suicide attacks in recent years, including the 2010 attack on the Moscow subway system and the 2011 assault on Domodedovo Airport.
However, no one has yet claimed responsibility for the bombings in the city once known as Stalingrad.
The attacks prompted false reports of other bombings in Volgograd and an evacuation of Moscow's Red Square after a woman left a package or bag there. St. Petersburg canceled a New Year's Eve fireworks display because of security concerns.
The attacks also drew attention to Russia's preparedness for the Olympics Feb. 7-23 in Sochi, the first Olympics in Russia since the breakup of the Soviet Union.
Putin, who has yet to comment on the attacks publicly, ordered security tightened across the country after meetings with government and security officials. He also sent Aleksandr V. Bortnikov, the director of the Federal Security Service, to Volgograd to oversee the investigation and security.
"I think we will be able to solve these crimes, particularly because we have some clues," Bortnikov said upon his arrival.
At least 12 people have been detained after a security sweep of the city was conducted, the Times said.
Vladimir I. Markin, a spokesman for Russia's Investigative Committee, said a man detonated a bomb with more than eight pounds of explosives Monday on the trolleybus full of morning commuters. In a statement, Markin said the bombs used in both attacks were similar and packed with shrapnel to make them more lethal. He cited the similarities as evidence the two attacks were linked.
"It's possible they [the bombs] were prepared in one place," he said.
Thomas Bach, president of the International Olympic Committee, condemned the attacks in a statement and also expressed confidence the Sochi Games would be secured.
The White House condemned the attacks and expressed sympathy for the victims' relatives while offering security support to Russia for the Olympics.
"We would welcome the opportunity for closer cooperation for the safety of the athletes, spectators and other participants," White House National Security Council spokeswoman Caitlin Hayden said.
Aleksandr D. Zhukov, president of Russia's Olympic Committee and Parliament's first deputy speaker, said all necessary security measures were being taken to protect athletes and visitors in Sochi, Interfax reported.
"No additional security measures will be taken in Sochi in light of the terrorist attack," Zhukov said. "Everything necessary has been done."
Pentagon spokesman Steven Warren told Britain's Guardian newspaper he was "not aware of any requests for assistance from either the Russians or the Olympic Committee."