Turkish police have detained 13 people in connection with the attack on Istanbul's Ataturk Airport, which killed 42 people, a Turkish official has said.
At least one of the detainees is believed to be a "foreigner", the official said.
"We're looking into the possibility of foreigners being involved," the source told Reuters.
"It is likely that at least one of them is a foreigner, but the investigation is still underway."
The detentions comes after police conducted simultaneous raids on 16 locations across Istanbul.
Police in Istanbul have conducted raids across the city targeting suspects from the so-called Islamic State (IS) militant group two days after attacks on Ataturk Airport, the state-run Anadolu Agency reported.
Officers searched several addresses in Istanbul's Pendik, Basaksehir and Sultanbeyli neighbourhoods, but it is not clear if any arrests were made, the agency said.
Authorities have blamed IS for Tuesday's coordinated attack, which has killed at least 42 people and injured more than 230.
Nine suspected militants, thought to have been in contact with IS members in Syria were detained in raids in three districts in Izmir, the news agency added.
The suspects are accused of financing, recruiting and providing logistical support to the Sunni hardline group.
Suicide bombers targetting Istanbul's airport incited panic to take lethal advantage in an attack that echoed the Brussels airport atrocity.Read the full story ›
US Vice President Joe Biden and his wife Jill signed a condolence book at the Turkish embassy in Washington for victims of the Istanbul airport attack.
Islamic State militants were suspected to have carried out Tuesday's attacks , which killed 42 people and wounded 239 at Europe's third-busiest airport.
He said the attacks on the airport had the effect of unifying the world against "distorted jihadists" but that defeating them would take some time.
I am absolutely convinced that combined with our resolve, we will wipe out the so called caliphate in Iraq and in Syria.
The world is united behind Turkey in our resolve.
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This CCTV image, published by Turkish media, claims to show one of the alleged attackers at Istanbul's Ataturk Airport moments before the terror attack.
US counter terrorism officials believe the group involved in the attack at Istanbul's Ataturk Airport contains "dozens more members" according to ITV News' Security Editor Rohit Kachroo.
Three suicide bombers attacked the airport on Tuesday killing at least 43 people and injuring over 230.
The death toll from the attack on Istanbul's Ataturk Airport has increased to 43, Turkish officials have confirmed.
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Some of the 41 victims of the terror attack at Istanbul's Ataturk Airport are being buried as Turkey observes a national day of mourning.