MOSCOW — Twin terrorist attacks in the city of Volgograd within 24 hours injected new urgency Monday into Russia’s long, ruthless effort to contain a diffuse Islamic insurgency on its southern border, one nominally led by a veteran, battle-scarred Chechen often called Russia’s Osama bin Laden.
The influence of the insurgent, Doku Umarov, seemed increasingly marginal until he surfaced in a video in July, ordering his followers to do whatever was possible to attack Russia as it prepared to be the host of the Winter Olympics in Sochi.
Although no one has claimed responsibility for the Volgograd attacks, Umarov’s threats, largely ignored at the time, suddenly seemed ominous, chillingly citing Russia’s transportation networks as potential targets.
On Monday morning, a suicide bomber gutted an electric trolley bus in Volgograd, killing at least 15 people and wounding dozens more. The bomb exploded during the morning rush hour on a street only a little more than 1½ miles from the city’s main railroad station, where a similar attack Sunday killed at least 17.
The attacks, coming only six weeks before the opening of the Olympics 400 miles away, sowed panic across the country. They prompted false reports of other bombings in Volgograd and the brief evacuation of Red Square in Moscow after a woman left a package near St. Basil’s Cathedral.
The attacks called into question Russia’s preparedness for an international sporting event that President Vladimir Putin and others intend to be a showcase of the country’s revival since the collapse of the Soviet Union.
Putin, who has made no public remarks since the first attack, ordered the tightening of security across the country after holding a series of meetings with government and security officials, according to the Kremlin. He dispatched the director of the Federal Security Service, Alexander V. Bortnikov, to Volgograd to oversee the investigation and enhanced security measures.
“I think we will be able to solve these crimes, particularly because we have some clues,” Bortnikov said after arriving there, without elaborating on the evidence investigators were pursuing.
He said that additional security had been deployed in public places, including the city’s transportation and energy facilities. At the same time, security officials in the city launched a security sweep that detained at least 12 people.
Vladimir I. Markin, a spokesman for the main national criminal investigation agency in Russia, the Investigative Committee, said a man carried out the second attack, detonating a bomb with more than eight pounds of explosives on Trolley Bus No. 15, which witnesses said was full of commuters.
The force of the blast tore the bus open, hurling bodies into the street and breaking windows in nearby five-story apartment buildings.
In a statement, Markin said the bombs used in both attacks were similar, packed with shrapnel to make them more lethal. He cited this as evidence that the two attacks were connected. “It’s possible they were prepared in one place,” he said of the bombs.
Monday’s attack was the third suicide bombing in Volgograd in recent months, in addition to at least two other attempted strikes, including a police station blast in August.
In October, a woman identified as Naida Asiyalova detonated a vest of explosives aboard a bus in the city, killing herself and six others.
Umarov has previously claimed responsibility for some of the most devastating suicide attacks in recent years, including ones that struck the Moscow subway system in 2010 and the city’s Domodedovo Airport in 2011.
Neither he nor his organization has claimed responsibility for the Volgograd bombings, and the police have yet to name any suspects.
The president of the International Olympic Committee, Thomas Bach, released a statement Monday condemning the attacks but expressing confidence that Russia would adequately secure the Sochi Games.
“I am certain that everything will be done to ensure the security of the athletes and all the participants of the Olympic Games,” he said.
Alexander D. Zhukov, the president of Russia’s Olympic Committee and the first deputy speaker of Parliament, said that all necessary security measures had been taken to protect athletes and visitors in Sochi.
His remarks did not address the threat outside of Sochi, however. With security already heavily tightened there, experts have warned that those who want to disrupt the Olympics might turn instead to “softer” targets elsewhere.
Russia steeled itself for more attacks, while residents in Volgograd expressed fear of taking any form of public transportation.