The three men who carried out this week's deadly attack on Istanbul's Ataturk airport were all from former Soviet Union states, Turkish sources say.
Turkish officials said one is understood to be from Russia's North Caucasus region and the others from Uzbekistan and Kyrgyzstan.
Turkey believes Islamic State (IS) was behind the suicide gun and bomb attack that left 44 people dead and about 240 injured.
Thirteen people have been arrested in connection with the attack.
The organiser of the attack has been named by Turkish media as Akhmed Chatayev, a Chechen believed to have acted as an IS recruiter, who is on an American counter-terror list.
Some agencies named another of the men as Osman Vadinov, said to have crossed into Turkey from the IS stronghold of Raqqa in Syria in 2015, although a police source has denied early reports the man was Chechen.
Family, friends and colleagues of victims gathered at the airport on Thursday for a memorial service as funerals continued.
One image on Turkish media purported to show the three men together at the airport moments before the attack, wearing dark jackets and carrying holdalls. Two are wearing caps, one is smiling.
IS has long recruited members from mainly Muslim parts of the former USSR, with Russian President Vladimir Putin putting the overall number at between 5000 and 7000 in October.
However, data published by the Soufan Group security consultants in December suggests the numbers are lower: 2400 from Russia and 500 apiece from Uzbekistan and Kyrgyzstan.
The government has made no official statement on nationalities yet and no one has said they carried out the attack on Tuesday evening.
Turkish Prime Minister Binali Yildirim said on Wednesday that "our thoughts on those responsible for the attack lean towards Islamic State".
Meanwhile, another Turkish official told AFP news agency: "Earlier today, the police raided 16 locations to detain 13 IS suspects, including three foreign nationals."
Turkish media said counter-terrorism police had raided several areas of Istanbul - including Pendik, Basaksehir and Sultanbeyli.
Arrests were also reported in the western coastal city of Izmir, where at least nine people were detained, accused of financing, recruiting and providing logistical support to IS.
Detailing the attack, Mr Yildirim said the three men had wanted to pass through the security system but on seeing the controls "took their weapons out of their suitcases and opened fire at random at the security check".
One attacker detonated his explosives downstairs in the arrivals terminal, Turkish officials said.
The second went upstairs and set off his explosives there while the third waited outside as passengers fled. He then detonated his explosives, causing the most casualties.
A Kalashnikov assault rifle, a handgun and two grenades were found on the bodies, Turkish media said.
Some 240 people were injured, dozens of whom remain in critical condition in hospital.
Dozens of anxious friends and relatives remain camped outside Istanbul's Bakirkoy hospital, waiting for news.
It is now known that of the 44 people killed, 24 were Turkish, three were Saudis and two Iraqis. In addition, China, Jordan, Tunisia, Uzbekistan, Iran and Ukraine each lost one citizen, and two Palestinians were killed.
Funerals began on Wednesday, including that of Muhammed Eymen Demirci, who landed a job on the ground services crew in May after a year unemployed, texting a friend saying "I got the job bro!"
He died waiting for a bus.
Tunisian doctor Fathi Bayoudh had reportedly been in Turkey for some weeks trying to secure the release of his son, who had been detained for allegedly joining IS.
Marvan Melhim and his wife, Nisreen, both work in Saudi Arabia, and had arrived with their three-year-old daughter.
"We heard shooting from a distance," said Marvan. "The explosion went off. I found my wife bleeding and my daughter too." Nisreen died in hospital shortly afterwards.
A friend of Serkan Turk said the physical education teacher had rushed to the site of the first explosion to help the wounded, but was killed by a later blast.