ISTANBUL -- Turkish investigators focused their attention Friday on a Chechen extremist, now thought to be the guiding force behind Tuesday's deadly attack at Istanbul's airport.
Akhmed Chatayev is an ISIS commander and a convicted arms smuggler. According to a U.S. official and Turkish media reports, he was also the mastermind of the massacre at Ataturk Airport.
Like thousands of other ISIS fighters, Chatayev is from Russia's Chechen minority and a veteran of Chechnya's brutal conflict with the Russian state.
His whereabouts are unknown. U.S. officials believe he's currently in Raqqa, the so-called ISIS capital.
Turkey's state news agency on Friday named two of the suicide bombers as Rakim Bulgarov and Vadim Osmanov. It also reported that Istanbul police arrested 15 foreigners in connection with the attack.
There are conflicting reports about where the bombers came from - including Russia, Uzbekistan and Kyrgyzstan -- but all are recruiting grounds for ISIS.
For five years now, foreign fighters have used Turkey as a transit country to reach the Syrian war zone. In 2013, CBS News watched people slipping through the border fence in broad daylight.
There's better security now, but Turkish politician Hursit Gunes told CBS News the airport bombing is partly a result of Turkey's failure to control its border.
"Some people call it a highway, some people call it a connection point," Gurnes said. According to him, the border has made Turkey a target.
The bombers reportedly used a cocktail of explosives in their suicide vests which are widely used in the Syrian conflict and readily purchased on the black market in Turkey.