>MOSCOW — A suspected female suicide bomber killed six people on a bus in Volgograd in what would be one of the biggest terror attacks in Russia since the 2011 blast at Moscow's Domodedovo Airport.

Twenty-eight people were hospitalized after the attack in Volgograd, about 900 kilometers (560 miles) southeast of the Russian capital, the Investigative Committee said on its website. Eight of the injured are in an intensive care unit, according to the law-enforcement agency in Moscow, which opened a criminal probe.

The bomber was tentatively identified as a 30-year-old native of the southern Russian region of Dagestan, the Investigative Committee said. Russian state television showed footage of a fireball shooting out the right side of the bus, recorded on a dashboard camera from a car behind the explosion.

"Attacks such as that in Volgograd illustrate the ability of regional militants to undertake attacks outside the North Caucasus republics themselves," Matthew Clements, chief analyst for Russia and the CIS at IHS Country Risk in London, said in an e-mailed note. "This is important in the context of the nearby February 2014 Sochi Winter Olympics."

Volgograd, once known as Stalingrad, is located less than 700 kilometers from Sochi, the Black Sea resort that will host next year's Winter Olympics. Russian federal troops have fought two wars after the collapse of the Soviet Union against separatists in nearby regions of the North Caucasus.

Chechen rebel leader Doku Umarov, who has claimed responsibility for three of Russia's deadliest terror attacks, released a video in July calling on Islamic militants to target the February Games. Umarov claimed responsibility for organizing the Domodedovo bombing.

A suicide bombing claimed by Islamic militants at Domodedovo, the busiest air hub in Russia, killed at least 37 people in January 2011. Dzhokhar and Tamerlan Tsarnaev, the brothers accused in April's Boston Marathon bombing, were immigrants of Chechen descent who had moved to the U.S. from Dagestan.

President Vladimir Putin stepped up security in the North Caucasus region near the Black Sea after the Tsarnaev brothers became suspects in the Boston Marathon bombing. Russia is collaborating with the law-enforcement agencies of more than 80 countries to identify possible threats to the Olympic Games, according to the Federal Security Service.

Putin is scheduled to meet muftis Tuesday to discuss cooperation between Russia's Islamic organizations and the state in Ufa, the capital of Bashkortostan, the Kremlin press service said by e-mail Monday.

Putin is receiving information about the Volgograd bombing from the special services, the president's spokesman, Dmitry Peskov, was cited as saying by Interfax. He declined to say if Putin has issued any orders in connection with the attack, according to the news service.

Officials from the Interior Ministry said earlier the blast may have been caused by faulty gas equipment on the bus, RIA reported. Investigators have discovered fragments of an explosive device at the site, the news service said, citing an unidentified law enforcement official.

"It was a really terrible scene," an eyewitness identified only as Ivan said in a telephone interview with state-run television station Rossiya 24. The bus was driving in an industrial area far from the city center when all of the windows and an escape hatch were blown out by the blast, said the man, who said he was driving behind the bus.

_ With assistance from Ekaterina Shatalova in Moscow.