Reuters

A bomb on a bus in Volgograd, Russia, on Monday killed at least 15 people. It was the second fatal blast in the city in two days.

MOSCOW — Twin terrorist attacks in the city of Volgograd within 24 hours injected new urgency on 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 attacks, coming only six weeks before the opening of the Olympics just 400 miles away, sowed widespread fear across the country. On Monday morning, a suicide bomber gutted a crowded electric trolley bus in Volgograd a little more than a mile and a half from the city's main railroad station, where a similar attack took place on Sunday.

The death toll in the two attacks continued to rise on Tuesday as victims died while being treated in hospitals. A Health Ministry spokesman told Interfax that 18 people had now died as a result of the bombing at the train station, while 16 died in the trolley bus attack.

The investigation into the bombings is just getting underway, but the attention of the Russian security services is already focused on the republic of Dagestan, which has become the hub of Muslim separatist violence in recent years, and on connections to the insurgent leader, Doku Umarov. He is a mysterious, almost mythical figure who fought in both Chechnya wars, which began nearly two decades ago and have come to symbolize the radicalization of a movement that began as a struggle for independence.

Mr. Umarov's influence had seemed to be waning in recent years, 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 attacks in Volgograd, Mr. Umarov's threats, largely ignored at the time, suddenly seemed ominous, chillingly citing Russia's transportation networks as potential targets.

Now, experts say, the question is whether the suicide bombings in Volgograd and one previous attack there are the first volleys in Mr. Umarov's promised campaign to disrupt the Olympic Games and discredit the government of President Vladimir V. Putin.

"The big question is will there be this sort of wave," said Gordon M. Hahn, a senior associate with the Center of Strategic and International Studies who has written extensively on Islamic terror groups in the Caucasus. "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."

The attacks prompted false reports of other bombings in Volgograd and the brief evacuation of Red Square here in Moscow after a woman left a package or bag near St. Basil's Cathedral. They also called into question Russia's preparedness for an international sporting event that Mr. Putin and others intend to be a showcase of the country's revival since the collapse of the Soviet Union.

Mr. Putin, who has made no public remarks since Sunday's 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, Aleksandr 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," Mr. Bortnikov said after arriving there, without elaborating on the trail of evidence investigators were pursuing. He said that additional security had been deployed at public places, including transportation and energy facilities in Volgograd, a city of about a million people formerly known as Stalingrad. 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 Monday's attack, detonating a bomb with more than eight pounds of explosives on trolley bus No. 15, which witnesses said was full of morning commuters. In a statement, Mr. Markin said the bombs used in both attacks were similar, packed with shrapnel to make them more lethal. He cited that as evidence that the two attacks were connected. "It's possible they were prepared in one place," he said of the bombs.