Reuters

At least 15 people were killed when an explosion ripped through a trolley bus in Volgograd on Monday.

MOSCOW — A deadly suicide bombing at a crowded railroad station in southern Russia on Sunday, followed by a blast in a trolley bus on Monday in the same city, raised the specter of a new wave of terrorism just six weeks before the Winter Olympics in Sochi.

President Vladimir V. Putin's government has worked to protect the Olympics with some of the most extensive security measures ever imposed for the Games. But the bombings, in Volgograd, underscored the threat the country faces from a radical Islamic insurgency in the North Caucasus that has periodically spilled into the Russian heartland, with deadly results, including several recent attacks.

Security has become a paramount concern at all major international sporting events, especially in the wake of the bombing at the Boston Marathon in April, but never before has an Olympic host country experienced terrorist violence on this scale soon before the Games. And would-be attackers may have more targets in mind than the Russian state.

Current and former American law enforcement and intelligence officials said Sunday that they were more concerned about security in Russia during the Sochi Games than they have been about any other Olympics since Athens in 2004.

Russian officials attributed the explosion on Sunday to a bomb packed with shrapnel, possibly carried in a bag or backpack. It was detonated in the main railroad station in Volgograd, a city 550 miles south of Moscow and 400 miles northeast of Sochi. The bomb blew out windows in the building's facade and left a horrific scene of carnage at its main entrance. At least 17 people were killed, and nearly three dozen others were wounded, some of them critically, meaning the death toll could still rise.

On Monday morning a second blast struck a trolley bus in the city, killing at least 15 people, including an infant, according to preliminary reports. Photographs posted by Russian news organizations showed that the force of the blast tore open the bus and shattered windows nearby. Nearly two dozen others were wounded in what officials immediately described as another suicide bombing.

The Sunday blast, captured on a surveillance video camera from across the central plaza in front of the station, occurred near the station's metal detectors, which have become a common security fixture at most of Russia's transportation hubs. That raised the possibility that an attack deeper inside the station or aboard a train had been averted.

Vladimir I. Markin, a spokesman for the main national criminal investigation agency in Russia, called the bombing an act of terror, though the exact motivation, target and perpetrator were not immediately clear.

Within hours of the attack, the authorities blamed a suicide bomber, and cited the gruesome discovery of the severed head of a woman, which they said could aid in identifying her as the suspect. Officials later said they had found a grenade and a pistol, and suggested that the attack might have been carried out by a man and a woman working together.

"Most likely, the victims could have been much higher if the so-called protective system had not stopped the suicide bomber from getting through the metal detectors into the waiting room, where there were passengers," Mr. Markin said in a statement on the agency's website.

The attack was the second suicide bombing in Volgograd in recent months. In October, a woman identified as Naida Asiyalova detonated a vest of explosives aboard a bus in the city, killing herself and six others.

In that case, the authorities said she was linked by marriage to an explosives expert working with an Islamic group in Dagestan, a republic in southern Russia where the police have struggled to suppress a Muslim separatist insurgency. A month later, the authorities announced that they had killed her husband and four others in a raid. But the attack on Sunday indicated that the threat was far from extinguished.

It was not clear why suicide bombers have chosen targets in Volgograd, a city of 1 million that was formerly called Stalingrad, the site of one of the crucial battles of World War II. It is the nearest major Russian city to the Caucasus, and its proximity may play a role.

Roman Lobachov, who was among those injured on Sunday, said he was at the station's security checkpoint when the blast occurred. "I passed through the metal detector, and at that moment there was an explosion," he said in remarks on the official Rossiya television network. He said he felt a shock like a blow to the head, "and I kind of dropped to the floor."

Mr. Putin vowed Sunday to redouble security at Russia's railway stations and airports, which are especially busy around the New Year's holiday.