Istanbul (CNN)As passengers at Istanbul's Ataturk Airport waited to board their flights Wednesday, workers picked up shattered glass and washed away blood that was barely dry.
Hours earlier, three terrorists attacked the arrivals hall and a nearby parking lot with gunfire and explosives, killing 41 people. Of the 239 people injured Tuesday night, 128 remained hospitalized Wednesday, officials said.
Despite the horror and carnage, "everything's quite calm right now, which is a little surreal as opposed to the scenes we saw here last night," witness Laurence Cameron said Wednesday.
Istanbul airport attacked despite heavy security
Istanbul airport attacked despite heavy security 03:01
"I was in the airport this morning looking for my lost luggage," he said. "They were sweeping up debris, and someone had hung up a big Turkish flag, pretty much right at the spot where (a) bomb had gone off -- sort of an act of defiance, which was quite moving."
It's not clear what motivated the terrorists, or if they had any idea whose lives they would take. At least 14 of the 41 killed were foreign nationals.
The attack killed six Saudis and wounded 27 more, the Saudi Arabian foreign ministry said.
The other victims killed included two Iraqis, one Tunisian, one Chinese, one Iranian, one Ukrainian, one Jordanian and one person from Uzbekistan, a Turkish official said. Three of the foreigners had dual Turkish citizenship.
There was no immediate claim of responsibility. But several officials said the attacks bear the hallmarks of ISIS.
Turkish Prime Minister Binali Yildirim said information from security forces suggests the terror group may be responsible, but authorities are still investigating.
"The terrorists came to the airport in a taxi and then carried out their attacks," Yildirim said. "The fact that they were carrying guns added to the toll. Preliminary findings suggest all three attackers first opened fire then detonated themselves."
That method is similar to mass shooting and suicide bombings at Paris' Bataclan concert hall last November. ISIS claimed responsibility for that attack.
Anatomy of an explosion 01:11
The initial thoughts among U.S. intelligence officials were that ISIS or an ISIS-inspired group was responsible, multiple U.S. officials said.
ISIS also has a history of airport attacks. In March, it claimed responsibility for dual suicide bombings at the main airport in Brussels, Belgium. At least 10 people died in those blasts.
And just like the Brussels attack, terrorists took a taxi to the airport.
The taxi driver who drove the terrorists in Istanbul was interviewed by police and later released, the Turkish state news agency Anadolu reported.
Passengers flee, hide at Istanbul airport 01:29
The cacophony of gunfire Tuesday night was quickly followed by the deafening blows of three explosions.
Sue Savage was seeing a friend off when the terrorists attacked. She said about 30 people were herded into a women's prayer room until authorities led them out and down an escalator into the main terminal hall.
Coverage from CNN Turk
"There was a lot of blood," she said. "There was so much glass on the floor, they were scuffing it aside so we didn't slip."
Istanbul Ataturk Airport
-- 11th busiest airport in the world (2015)
-- Europe's third busiest airport (2015)
-- ~62 million passengers passed last year
-- There are two main passenger terminals
-- Terminal 1: older, smaller, domestic
-- Terminal 2: newer, bigger, international
-- Vehicle checkpoint at compound entrance
-- X-ray checkpoint at terminal entrance
-- The airport is roughly 15 miles from the city
Source: Airports Council International/CNN
Of the three bombers at the airport, two were at the international terminal, and the third terrorist was in the nearby parking lot, a Turkish official told CNN.
The assailants have not been identified, but there is a "strong suggestion that they are foreign," a senior Turkish government source told CNN.
The attacks happened on a warm summer night at the airport, east of Istanbul, which is the 11th busiest in the world in terms of passenger traffic.
Ataturk Airport is "one of the most secure airports in the world," CNN senior law enforcement analyst Tom Fuentes said. But the airport has been "very overwhelmed for several decades with terrorism from PKK."
Experts say Turkey is especially vulnerable because of the terrorists operating there.
"You cannot protect these airports 100% ... especially in a place like Turkey, where ISIS has cells everywhere," said retired Lt. Col. Rick Francona, a former U.S. military attaché in Syria.
As part of the U.S.-led coalition against ISIS, Turkey allows coalition planes to fly raids on ISIS targets in neighboring Iraq and Syria from its territory.
And last year, Turkey resumed hostilities with the PKK, Kurdish militant separatists, after a two-year cease fire broke down.
The PKK has been in an armed struggle with the Turkish government for decades and is considered a terror group by Turkey, the United States and the European Union.
But there's been a shift in strategy recently, according to Francona, the former U.S. military attaché in Syria.
"Over the last couple of months, the Turks have really changed their focus from only the PKK to going after ISIS as well," he said. "Remember when we first started this exercise, the Turks were giving a lot of lip service to going after ISIS when really they were trying to marginalize and isolate the PKK."
ISIS also promised an uptick in attacks during the holy month of Ramadan, which is nearing its end.
The previous attacks
Turkey has been rocked by a string of terror attacks over the past year as it weathers bombing campaigns by ISIS and Kurdish militants.
The Tuesday attack in Istanbul is the eighth suicide bombing in Turkey so far this year. To date, at least 140 people have been killed.
Understanding Turkey's deadly catalog of enemies
The violence has had an impact on Turkey's tourism industry, a key sector of the national economy.
Turkey: A country in turmoil 01:09
Turkish President Recep Tayyip Erdogan called for a unified international fight against terrorism following the attack.
"Make no mistake: For terrorist organizations, there is no difference between Istanbul and London, Ankara and Berlin, Izmir and Chicago or Antalya and Rome," he said. "Unless all governments and the entire mankind join forces in the fight against terrorism, much worse things than what we fear to imagine today will come true."
If ISIS is responsible for the attack, Soner Cagaptay of the Turkish Research Program at the Washington Institute believes that Turkey "will retaliate with full war."
"I would expect that Turkey's vengeance will come down like rain from hell," he told CNN. "For Turkey now, fighting the so-called Islamic State is going to be priority number one."
And that involves getting involved in the chaotic, messy conflict that's ravaged Turkey's neighbor, Syria, since 2011.
Francona described Syria as "where the nexus of every problem there is in the Middle East comes to roost."
The Turkish military said its artillery and U.S.-led coalition forces hit 15 ISIS positions in northern Syria Tuesday, according to Anadolu.
It's not clear if the strikes came before or after the attack.