In the video, the green-and-white bus ambles slowly along a tree-lined street in the Russian city of Volgograd. Moments later it is engulfed by a plume of smoke and orange flames, nearly toppled over, and its windows are blown out.

The explosion Monday, Russian authorities said, was caused by a suicide bomber whose detonation killed herself and at least six other people. A Russian government investigative committee said preliminary indications were that the attacker was a woman from the North Caucasus region of Dagestan, adjacent to Chechnya.

Suicide bombings have become less common throughout Russia than they were a decade ago. The timing and location of this blast, however, is potentially worrisome as Russia gears up for the spectacle of the 2014 Winter Olympics, set to begin in less than four months.

The Kremlin has invested billions of dollars preparing the Black Sea resort city of Sochi for the February games, the first Olympics hosted by Russia since the 1980 summer games. It has also spent untold sums to protect the games from terrorist threats. Though on the Black Sea, Sochi is a relatively short distance from the North Caucasus, where the two Chechen wars of the 1990s and the 2000s have given way to a stubborn, festering insurgency that defies Kremlin efforts to stamp it out.

Volgograd, the Volga river industrial city that was previously know as Stalingrad, is well outside the North Caucasus: about 380 miles north of the Chechen capital, Grozny. The Investigative Committee, a rough Russian equivalent of the FBI, identified the woman who detonated the bomb as a native of Dagestan, a violent province where slain Boston Marathon bomber Tamerlan Tsarnaev spent six months in 2012. A witness said the woman boarded the bus around 2 p.m. local time, wearing a traditional Islamic headscarf and detonated the bomb almost immediately, the committee said. At least 28 people were hospitalized, eight critically.

Video captured by a dashboard-mounted video camera showed the bus exploding in a puff of fire and smoke as it accelerated on the tree-lined road. Passengers scrambled out of blown-out doors and windows after it had stopped.

There was no immediate claim of responsibility for the attack. The ITAR-Tass news agency quoted law enforcement official as saying the woman was married to a man who was member of a militant group in Dagestan.

President Vladimir Putin has put his personal reputation on the line for Russia to host the upcoming Olympics. He personally lobbied the International Olympic Committee to award Russia the games, and has regularly upbraided government officials for the astronomical sums being spent. By some accounts, the Sochi Olympics will cost upwards of $50 billion, the most expensive on record.

Sochi was awarded the games with reassurances from Russian officials that the security concerns that have plagued the North Caucasus for decades would not threaten athletes or spectators.

Sochi is also located just a few miles from the border with the Georgian region of Abkhazia, which was the focus of a civil war in the early 1990s. In 2008, the region was invaded by Russian forces during a short war with Georgia and Abkhazia has been recognized as an independent nation by just a handful of countries.

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Long plagued by poverty, unemployment and corruption, the North Caucasus descended into chaotic violence with the first Chechen war of the 1990s. The conflict later evolved from a local fight for independence into a battleground for jihadists with an influx of foreign fighters. With the second war that began in 1999, suicide bombings became a regular feature, in particular ones carried out by the wives of men killed fighting Russian forces, who became known as “black widows.”

Suicide bombers backed by Islamist militants were behind the 2001 attack at a Moscow airport that killed 37 people, and twin bombings that killed 40 people on the Moscow subway in 2010.