THREE years ago, they met and fell in love on a Russian dating website. Three days ago, this "dashing" young man helped strap the explosive belt to his wife which blew her up - along with five others - on a public bus.

Islamic bomber Naida Asiyalova, 30, and Dmitry Sokolov, who was eight years her junior, were on Russia's most-wanted list - and he is now on the run while her remains lie in a body bag.

A picture is emerging of a couple from very different backgrounds, drawn together by a mutual fanaticism, who urged each other on to ever greater deeds of evil. She introduced him to her fellow terrorists, while he became an expert in their ways - and sent his wife on her final murderous mission.

It was just days before her birthday - October 25.

Monday's attack on a bus near the southern city of Volgograd, which also injured 30 people, was the latest horror in a terror war being waged against Russia by Islamist fighters from the North Caucasus regions of Dagestan and neighboring Chechnya.

Rasul Temirbekov, a spokesman for the Investigative Committee's regional branch in Dagestan, said Asiyalova met Sokolov, a university student, in Moscow and recruited him to join the rebels in her native Dagestan.

Along the way he converted and received the Islamic name Abdul Jabbar but was also known under the nickname "Giraffe."

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Dmitry developed an interest in explosives and studied Arabic. Then he dropped off the security radar. Sokolov has been on the run since he left his home in a Moscow suburb in the summer of 2012, according to the investigators.

One newspaper said Sokolov, who is believed to be behind other attacks, likely made Asiyalova's suicide belt.

The bomb is believed to have contained about 500-600 grams of TNT equivalent and was filled with metal objects and dowel pins.

Russian officials said last night they were now looking for him in Dagestan as well as Moscow.

Investigator Temirbekov said that police didn't know if the detonation was planned, or if Asiyalova "changed her plan on her way, looking for the most crowded places."

"Having bought a ticket to Moscow, Asiyalova was going on a bus from Makhachkala and stopped in Volgograd. Her left hand was in plaster, and she was holding it with her right hand. Almost on the outskirts of the city, Asiyalova left the bus and returned to the Volgograd city center," the police spokesman said.

Investigators said they believed Asiyalova had a fatal bone illness, but her mother disputed this.

She told the daily Izvestia newspaper that her daughter had stomach problems after taking diet pills - 'but nothing serious'.

Ravzat Asiyalova also said her daughter became strongly religious three years ago. She told Izvestia she disapproved of that and her daughter rarely called her, mainly to avoid arguments.

The mother's home has since been seized and sealed by Russian authorities

Russian state television released pictures of the attacker's passport that looked different, even though they carried the same number.

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 said a photo of Asiyalova in a headscarf had been put atop the scan by a security agent.

Most of the passengers killed and wounded on the bus were students coming home after lessons.

Dmitry Yudin, a student who suffered a concussion and arm wound, said he had noticed Asiyalova when she boarded the bus because she wore a dark Islamic headscarf. He told media the suspected attacker looked "calm and collected" and kept a low profile.

"Those who died were ordinary people, they were innocent. It's a terrible tragedy. Anyone of us could be in their place. No one thought that anything of this kind could ever happen in our city," Ivan Skorobogatov, a city resident, told Itar-Tass.

Monday's blast was the deadliest attack outside the volatile North Caucasus in several years. An Islamic insurgency has been simmering in the region for more than a decade after two separatist wars in Chechnya.

Female suicide bombers, known as "Black Widows" are a feature of the Islamist Chechen terror campaign. The women are often the widows of Chechen fighters killed in their struggle against Russian occupation.

Willing to become martyrs for their beliefs and their lost loved ones, the Black Widows achieved international infamy in October 2002 when several of them participated in a raid on a Moscow theatre.

The militants want to establish an Islamist state across the North Caucasus. In July, guerrilla commander Doku Umarov vowed to resume attacks against civilian targets after a brief hiatus, and to also target the February 7-23 Sochi Games.

Volgograd lies 650km northeast of the North Caucasus.

In Dagestan, the center of the insurgency, bombings and shootings occur almost daily. The Tsarnaev brothers, accused of carrying out the Boston Marathon bombings, have roots in Dagestan and Chechnya.

Russian security services also suspected Sokolov of making explosives that wounded 15 people in two attacks in the Dagestani city of Makhachkala earlier this year.

The lifenews.ru website, which relies extensively on security services sources, said both Asiyalova and Sokolov were listed on the federal wanted list.