The man Russian police believe was the suicide bomber who killed 14 people in a blast on the St Petersburg metro this week developed an interest in Islam and soon after traveled to Turkey, two people who know him told Reuters.
The two people said they did not know for sure if the man, Akbarzhon Jalilov, went on from Turkey to neighboring Syria. Turkey has been routinely used by radical Islamists as a route into areas of Syria controlled by the Islamic State group.
If Jalilov had been in Syria, that would expose a major gap in Russia's counter-terrorism procedures, which rely heavily on identifying anyone who has been with militants in Syria and stopping them from returning to Russia, or arresting them.
The metro blast happened on Monday afternoon just as Russian President Vladimir Putin visited St Petersburg. No group has claimed responsibility. But Islamic State has threatened acts of violence on Russian soil in reprisal for the Kremlin's military intervention in Syria in support of President Bashar al-Assad.
Jalilov, the suspected suicide bomber, was born in 1995 and grew up in the city of Osh in Kyrgyzstan, a mainly Muslim ex-Soviet Republic in Central Asia.
Around 2011 he moved to St Petersburg. He worked in low-paid jobs for several years. Photographs posted on social media showed a stylishly dressed young man. His online posts offered no hint of any ties to Islamist militants.
But there is a gap in Jalilov's biography from the end of 2015 until the start of this year. During that period several acquaintances said he disappeared from view.
He reappeared when he visited Osh in February this year. In March he returned to St Petersburg and rented an apartment from where he set off on the day of the bombing carrying a rucksack and a bag.
By speaking to several people who knew Jalilov well, Reuters has been able to piece together a picture of his life in the missing years.
According to someone from Osh who worked as a cook alongside Jalilov in a St Petersburg restaurant in 2014, he was an even-tempered young man who did not drink or use curse words.
"I would have said firmly that Akbar was not capable of doing anything bad," said the source, using an abbreviation of Jalilov's name. The source asked not to be identified because he did not want the authorities to associate him with a suicide bomber.
The two worked in the same outlet of the Sushi Wok restaurant chain.
During the course of 2014, said the former work colleague, Jalilov developed an interest in Islam. He prayed, went to the mosque, read the Koran, and started growing a beard. But the source said he did not notice any signs of extremism.
The second source, another native of Osh who also worked with Jalilov in St Petersburg, said he too was aware that Jalilov had acquired an interest in religion. But he said Jalilov never tried to push his faith onto anyone else.
During 2015, said the first source, Jalilov left St Petersburg. The source said he heard from other cooks at the restaurant that before leaving, Jalilov had said he was going to Turkey.
He said, according to the source, that "there are good jobs in Turkey, and things aren't working out (with work) in St Petersburg."
The second source, who also did not want to be identified, told Reuters he knew from Jalilov that he went to Turkey. The source said Jalilov had traveled there in November 2015, to join his uncle who was living in the Turkish region of Antalya.
Contacted by Reuters in Osh, Jalilov's uncle, Khasan Kuchkarov, told Reuters he had lived in Antalya but left in September 2015 and was unaware of Jalilov traveling there.
After Jalilov's departure from St Petersburg, there was talk among his old work colleagues, and among people who knew him back in Osh, that he had gone to Syria, both of the sources said. But neither of them knew for sure if he was in Syria, they said.
Officials in Russia's Investigative Committee, the state body investigating the metro bombing, and in the Federal Security Service, declined to comment on whether Jalilov traveled to Turkey or Syria.
Russian law enforcement agencies have arrested eight people on suspicion of involvement in the metro bombing. Six were detained at an apartment on Tovarishcheskii Avenue in a residential suburb of St Petersburg, and two in Moscow.
In the apartment, law enforcement officers found an explosive device identical to one which, police say, Jalilov had left at a metro station before going on to blow up the carriage in another part of the subway system. The device he left, hidden inside a fire extinguisher, did not go off.
At a court arraignment hearing on Friday, only one of the detainees, Ibrahibzhon Ermatov, 24, said he knew Jalilov. He said they had worked together in a Sushi Wok branch in Vsevolozhsk, a town near St Petersburg. Ermatov denied being a supporter of Islamic State.
The first of the two sources who knew Jalilov said the apartment on Tovarishcheskii Street was used by some people as a temporary stopping off point for workers arriving in St Petersburg from Osh.
He said new arrivals found out about the 32 square-meter apartment via friends, and there was a regular turnover of tenants. Neighbors confirmed that to Reuters. The source said he had once stayed there himself for a day before moving on to longer-term accommodation.
The source said Jalilov had not stayed at the apartment, to his knowledge. He said he knew one of the apartment tenants who had been arrested, Dilmurod Muildimov, from back home in Osh. Muildimov also worked for the Sushi Wok restaurant chain.
The source said Muildimov had arrived in St Petersburg a few months ago, and that he did not know Jalilov.
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