Russian security forces hunted on Tuesday for the husband of a bomber who blew herself up on a bus in southern Russia, killing six people and wounding more than 30. They also raised the possibility that Moscow, not Volgograd, was the bomber's original target.

Investigators say 30-year-old Naida Asiyalova, a native of the volatile province of Dagestan in Russia's North Caucasus region, was married to an ethnic Russian man who had joined Islamic militants. They say her husband, Dmitry Sokolov, has become a top rebel expert in explosives and could have been involved in equipping his wife for Monday's suicide mission.

Sokolov has been on the run since he left his home in a Moscow suburb in the summer of 2012, according to the investigators.

The bombing in the southern Volgograd region was the first attack against a civilian target outside the volatile North Caucasus region in years, raising fears of a new wave of terror just three-and-half months before the start of the 2014 Winter Games in Sochi.

Soon after the attack, Russian state-controlled TV stations showed pictures of Asiyalova's passport which contained a black-and-white photograph of her wearing an Islamic headscarf, in violation of Russian regulations. Some observers noted that photo would have led to her detention, if police had stopped her for an identity check. The shown passport looked entirely intact, which seemed unusual following the deadly suicide explosion.

On Tuesday, state television released a new picture of her passport that had a different colour photograph without a headscarf and looked damaged. The passports shown Monday and Tuesday had identical numbers, and bloggers and online media were quickly abuzz with conspiracy theories.

The Investigative Committee, the powerful Russian agency conducting the probe, said it hadn't released the first picture, only the second one shown on Tuesday.

NTV, one of the three state-controlled nationwide TV stations, said Monday's picture was a scan of Asiyalova's passport taken from her personal dossier at Russian security agencies that had been monitoring her for her suspected terror links.

It was not clear why Asiyalova chose Volgograd, since she had a ticket for Moscow, authorities said.

Vladimir Markin, the spokesman for the Investigative Committee, said authorities are trying to determine whether Asiyalova had planned her attack in Volgograd or made an impromptu choice along the way. He said Asiyalova took a Moscow-bound bus from Dagestan, but left it in Volgograd and took a local bus, where she detonated her explosives Monday.

The bomb was rigged with shrapnel, which caused severe injuries and left many of the wounded in serious conditions. Most of the passengers were students coming home after lessons.