MOSCOW (AP) -- Although there has been no claim of responsibility for the two bombings in Volgograd that killed at least 31 people in less than 24 hours, suspicion quickly fell on Caucasus rebels who have tormented Russia with terrorist attacks for nearly two decades.
The insurgency has its roots in wars fought between separatist Chechen rebels and Russian forces, but it has spread throughout the North Caucasus region, which includes several mostly Muslim republics.
Most assaults have been within the Caucasus, but attackers have occasionally reached far outside the region, including Moscow. Volgograd, which borders the North Caucasus region to the north, was earlier hit by an insurgent suicide bombing that killed six people in October.
A look at the insurgency:
A full-scale war in Chechnya with Russian forces began in late 1994. Although the Russians inflicted enormous damage, the separatist rebels fought them to a standstill. In the fall of 1996, the army withdrew.
The violence of the first war was largely confined to that small republic, but rebels ventured into other parts of Russia. In 1995, militants seized hostages at a hospital in the city of Budyonnovsk. At least 129 civilians were killed, many of them as Russian forces seized control of the hospital.
When Russian forces pulled out, Chechnya became de facto independent and fell into lawlessness and near anarchy, becoming a breeding ground for fighters who increasingly took on a specifically Islamist ideology.
In 1999, fighters invaded neighboring Dagestan with the goal of establishing an Islamic caliphate.
They were repelled, but the invasion touched off the second war between Russian forces and rebels in Chechnya. Terrorist attacks spread well beyond the region, including the 2002 seizure of a theater in Moscow while a musical was being performed. All 40 rebels and about 130 hostages died after Russian forces pumped narcotic gas into the theater to end the siege.
In 2004, insurgents seized a school in the town of Beslan. Some 380 people died in explosions and shooting as Russian forces tried to take control. A suicide bomber killed 37 people in January 2011 at Moscow's Domodedovo Airport, the country's busiest.
As the insurgency expanded its operations, concentrating on Dagestan as the second Chechen war subsided, it became known as the Caucasus Emirate movement. Since 2007, its leader has been Doku Umarov, formerly the president of the self-proclaimed separatist Chechen Republic of Ichkeria.
The command structure is unclear. With adherents scattered across a large area, much of its activity may be directed by local warlords whose influence is limited to a particular district, some analysts say.
After the Domodedovo bombing, Umarov declared an end to attacks on civilians, but reversed that order in July, urging his men to "do their utmost to derail" the Sochi Olympics, which he described as "satanic dances on the bones of our ancestors."
EFFORTS TO QUELL
Under Kremlin-backed leader Ramzan Kadyrov, Chechnya has quietened. A huge infusion of federal funds has turned parts of ruined Grozny, the capital, into a shiny display of new buildings. But Kadyrov is widely denounced for human rights abuses, including allegations of killing opponents. He has also imposed some Islamic restrictions on the region, including mandatory public headscarves for women.
Dagestan has become the epicenter of the violence, appearing to be effectively out of the central government's control. Bombings and attacks on police there are near daily occurrence, as are police operations that corner suspected insurgents and almost always end in the rebels' death.
Attacks also occur less frequently in the republics of Ingushetia, Kabardino-Balkariya and Karachayev-Cherkessiya.
In 2010, as concern about the rebellion grew in the run-up to the Sochi Olympics, Russia created a new administrative district encompassing the Caucasus republics to try to coordinate efforts to wipe out the insurgency. The district's center is the city of Pyatigorsk, where a car bombing near a police station killed three people on Friday.