ISTANBUL -- Turkish police swept into 16 different locations across the capital city on Thursday, taking 13 people into custody on suspicion of involvement in the deadly attack on Istanbul's airport.
CBS News correspondent Holly Williams reports that consensus is building that the Islamic State of Iraq and Syria (ISIS) was behind the attack that left 44 people dead and at least 230 others wounded, and we now have a much clearer idea of how it was carried out.
It began when the three attackers arrived at the international terminal of the busy Ataturk international airport together in a taxi -- armed with automatic weapons, grenades and suicide vests -- and then split up.
Those tactics alone, according to U.S. intelligence officials, suggest ISIS involvement. CIA Director John Brennan didn't officially assign blame on Wednesday, but he came close, saying the carnage "certainly bears the hallmarks of ISIL's depravity," using an alternate acronym for the group.
CBS News has learned that there had been ISIS "chatter" in recent months indicating that the group was intent on targeting transportation hubs in Turkey -- including Ataturk Airport.
A U.S. intelligence source said Turkish investigators had located the taxi driver who dropped the attackers off at the airport, who said he didn't recognize the language the men in his cab were speaking.
U.S. sources have confirmed to CBS News that the attackers were not Turkish nationals, saying at least one of them was likely from the restive Russian region of Chechnya.
Turkish officials told news agencies on Thursday that the bombers were from Russia, Uzbekistan and Kyrgyzstan, but American officials did not immediately confirm that information.
DNA tests are underway to confirm the attackers' identities.
There were further unconfirmed reports Thursday bolstering the theory that at least one of the attackers was from Russia, and may have traveled to Turkey from the ISIS stronghold of Raqqa in Syria. Turkey's southern border is shared with Syria.
Turkish media have published an image from a security camera that they say shows the three bombers arriving.
Williams says two of the attackers managed to force their way inside the building, armed with guns and hand grenades, despite a first security perimeter at the terminal door.
Reports in Turkish media say one of the bombers can be seen on security cameras dressed in a thick, quilted jacket -- in the middle of summer -- which should have aroused suspicion.
One of the bombers detonated his explosives in the middle of the busy arrivals area, another at a nearby entrance, and the third upstairs in the departures section, close to passport control.
Turkish security personnel managed to shoot that last attacker, leaving him sprawled on the ground before he too blew himself up.
"I heard the blast, so I thought there must be some bomb, because then (I heard) shooting," Martin Kemper, from Atlanta, Georgia, told CBS News. He was just a short distance away from that blast, waiting for a connecting flight.
The attack sent him and other panicked passengers running for a place to hide.
"It was terrifying, especially as you have these images, the bomb went off -- ok, you're still alive, but now they come and shoot you -- because you think of Paris, Orlando, you know, all this what you have seen," Kemper said.
Heartbroken relatives began burying the dead on Wednesday. Most of those killed were Muslims, murdered by terrorists during the Islamic holy month of Ramadan.
The attackers were not Muslims, said Turkish President Recep Tayyip Erdogan, adding that through their actions they "prepared their place in hell."