Turkish police arrested 13 people in connection with the deadly attack on Istanbul's airport, officials said Thursday.
More than 40 people died and over 200 were injured when assailants with guns and explosives hit the airport on Tuesday.
Officials have said the coordinated assault on Ataturk airport bore the hallmarks of ISIS, but there has been no official claim of responsibility.
Anti-terror 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 have been transferred to police headquarters while another three individuals are currently being searched for, Anadolu reported.
Turkish officials have not publicly identified the attackers. Police sources told NBC News that assailants were Russian, Uzbek and Kyrgyz.
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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.
Separately, nine people with suspected ties to ISIS were arrested in the coastal Izmir early Thursday, according to Anadolu Agency, which said 200 police officers participated in the operation. It was not immediately clear if those arrests were in relation 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 ongoing insurgency from the Kurdistan Workers' Party, or PKK.
Turkey's military said Thursday that its fighter jets had struck PKK targets in southeastern Turkey and northern Iraq a day earlier. The state-run Anadolu Agency 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.
Around 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 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 Turkish national anthem.
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Outside of the terminal around a dozen ground handlers gathered after the memorial, waiting to head to a colleague's funeral.
"We're all still in shock," one told NBC News.