Turkish police have arrested 13 people in connection with the deadly attack on Istanbul's airport, officials said Thursday.
More than 40 people died and more than 200 others were injured when assailants with guns and explosives hit the airport Tuesday.
Officials have said the coordinated assault on Istanbul Ataturk Airport bore the hallmarks of ISIS, but there has been no official claim of responsibility.
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Anti-terrorism police carried out 16 raids in relation to the Ataturk attack overnight. Interior Minister Efkan Ala told Turkey's Parliament that 13 people — including four foreigners — had been arrested.
Those suspects were transferred to police headquarters while three other people individuals were being sought, the Turkish news agency Anadolu reported.
Turkish officials haven't publicly identified the attackers. Police sources told NBC News that the assailants were Russian, Uzbek and Kyrgyz.
Up to 2,000 Central Asians are fighting for ISIS in Syria and Iraq, U.S. intelligence sources recently told NBC News on condition of anonymity.
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Separately, nine people with suspected ties to ISIS were arrested in the coastal city of Izmir early Thursday, according to Anadolu, which said 200 police officers participated in the operation. It wasn't immediately clear whether those arrests were related to the airport attack.
The country is grappling with a number of enemies domestically and abroad — participating in the U.S.-led coalition against ISIS while battling an insurgency from the Kurdistan Workers' Party, or PKK.
Turkey's military said its fighter jets struck PKK targets in southeastern Turkey and northern Iraq on Wednesday. Anadolu reported that eight targets were destroyed.
The operations came as the death toll climbed to 44 on Thursday — including 19 foreigners — and funerals continued for the victims.
About 90 of the more than 200 people injured in the attack remained hospitalized, the Istanbul governor's office said in a statement.
Hundreds of people gathered for a moment of silence and an emotional memorial at the airport. Check-in screens showed the image of a black ribbon, and a large table covered in black cloth bore framed pictures of the victims.
After the prayer, those gathered threw red carnations onto the impromptu shrine before breaking into the national anthem.
Outside the terminal, about a dozen ground handlers gathered after the memorial, waiting to head to a colleague's funeral.
"We're all still in shock," one of them told NBC News.