Turkish police have detained 11 foreigners suspected of belonging to an Isis cell linked to the attack on Istanbul’s main airport, state media reported.
Forty-four people were killed in Tuesday's bombings and shootings, which targeted one of the world's busiest airports. The three suspected attackers were Russian, Uzbek and Kyrgyz nationals, a Turkish government official has said.
The pro-government Yeni Safak newspaper has said the organiser of the attack, the deadliest in a series of suicide bombings in Turkey this year, was suspected to be a man of Chechen origin called Akhmed Chatayev.
Chatayev is identified on a United Nations sanctions list as a leader in Isis responsible for training Russian-speaking militants.
He was arrested in Bulgaria five years ago on a Russian extradition request, but freed because he had refugee status in Austria, a Bulgarian judge said. A year later he was wounded and captured in Georgia but again released.
Friday's dawn arrests by counter-terror police in the European side of Istanbul brought the number of people detained in the investigation to 24, the state-run Anadolu Agency said. A police spokesman could not confirm the report.
Turkish officials have not given many details beyond confirming the attackers' nationalities. They have previously said that forensic teams were struggling to identify the suicide bombers from their limited remains.
Yeni Safak has said the Russian bomber was from Dagestan, which borders Chechnya, where Moscow has led two wars against separatists and Islamist militants since the Soviet Union collapsed in 1991.
Turkey’s Hurriyet newspaper named the Russian bomber as Osman Vadinov and said he had come from Raqqa, the heart of Isis-controlled territory in Syria. The Russian interior ministry has said it was checking information about Vadinov.
In 2012, Georgian officials said Chatayev had been wounded in a special forces operation against an unidentified group in the remote Lopota Gorge near the border with Dagestan. The group was believed to have been made up of Russian Islamist insurgents fighting against Moscow's rule in the North Caucasus.
Chatayev, whose foot was later amputated due to his injuries, was arrested on charges of weapons possession. He denied this and said that he had been sent to the gorge as a negotiator at the request of Georgian officials.
He was released on the orders of a Georgian court later that year and cleared of all charges in January 2013. "He was released lawfully, whether it was a mistake or not," former Georgian interior minister Vakhtang Gomelauri said this year.
In 2011, he was detained by Bulgarian police at the country's border with Turkey as Russia wanted him for “participation in an armed group and for the recruitment of persons for terrorism and for financing terrorism”, a Bulgarian judge told Bulgarian national radio on Friday.
However, the Bulgarian court refused to extradite him, saying his refugee status, which had been granted to him in Austria in 2003, remained valid in all countries that are signatories to the Geneva Convention, which includes Bulgaria.
Separately, Turkish security forces detained four Turkish citizens on Wednesday at the Oncupinar border crossing with Syria on suspicion of membership of a terrorist group, the local governor's office said in a statement.
The four were attempting to return to Turkey from a conflict zone in Syria under Isis control, it said.
Additional reporting by Reuters
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