ISTANBUL — (CNN) As passengers at Istanbul’s Ataturk Airport waited to check in and board their flights Wednesday, workers picked up shattered glass and washed off blood that was barely dry.

The airport is up and running hours after the Tuesday night attack, which killed at least 36 people and injured 147 others, Turkish officials said.

One Ukrainian is among those killed, according to the country’s foreign ministry.

And an Iranian citizen was also killed, according to the nation’s semiofficial ISNA. Identities of the other victims have not been released.

The country has since canceled all flights to Istanbul, according to the Iranian Tasnim News Agency.

‘Bloody boot marks’

Gunshots, screams and explosions pierced the air during the attack Tuesday.

Witnesses described deadly carnage and crowds in a panic as the attackers struck one of the world’s busiest airports.

Sue Savage was in the airport to see a friend off when she heard gunshots and then an explosion. More gunshots rang out, followed by another boom, she told “Erin Burnett OutFront.”

She and 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.

“There was a lot of blood,” she said. The floor was marked with bloody boot marks as the group was led to the other side of the room.

“There was so much glass on the floor they were scuffing it aside so we didn’t slip,” she said.

Video from inside the terminal shows people fleeing and the bright orange flash of fire from one of the explosions. Victims stagger and some fall from the blood-covered, slick floor.

Another video shows a man walking with a gun, which he drops when he’s apparently shot by a security officer. The man slumps to the ground and the officer briefly stands over him before running. About 10 seconds later, there is an explosion.

Traveler Laurence Cameron described what he saw after he stepped off a plane.

“It was just a massive crowd of screaming people. Some were falling over themselves. A poor chap in a wheelchair was just left, and everyone just rushed to the back of the building, and then people ran the other way and no one really seemed to know what was going on,” he told CNN. “Where you normally hail a taxi, that is where the attack happened. The ground is just kind of shredded. There are bloodstains on the floor as well.”

The three bombers

Of the three bombers in the airport attack, two were at the international terminal, and the third terrorist was in the nearby parking lot, a Turkish official told CNN. All three detonated suicide vests.

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 terror groups 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 attache in Syria.

The airport was closed overnight for several hours, and flights into the airport had been diverted to the capital of Ankara and other cities.

It has since reopened and flights have resumed.

Terrorists came in a taxi

There was no immediate claim of responsibility.

“The terrorists came to the airport in a taxi and then carried out their attacks,” Prime Minister Binali 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.”

Yildirim didn’t clarify why, but said signs pointed to ISIS being behind the attacks.

Multiple U.S. officials told CNN’s Pamela Brown and Elise Labott that the early thinking among U.S. intelligence officials was that ISIS or an ISIS-inspired group was to blame. Like the Turkish Prime Minister, the officials said there is much more investigating to be done.

One official said the attack bears the hallmarks of ISIS because of the target and the method. And a senior US official pointed to the way it was coordinated using weapons and explosions.

Some analysts say that this type of attack is a response to ISIS recent defeats in Iraq and Syria, notably the loss of Falluja.

“They may lose ground, but they’re still a potent force,” said CNN analyst Bob Baer, a former CIA operative.

It would be a “big surprise” if it was the PKK — or Kurdistan Workers’ Party — based on how the attack was conducted. The PKK tends to target military and security installations, other officials said.

The timing

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 attache 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 past 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.

In March, at least 37 were killed when a car bomb detonated near a bus stop in the Turkish capital, Ankara, in an attack claimed by a Kurdish militant group.

A month earlier, 28 were killed in a blast targeting military vehicles in central Ankara. A Kurdish group claimed responsibility.

And in January, at least 10 German tourists died in a suicide bombing in Istanbul’s Sultanahmet Square that Turkish authorities linked to ISIS.

The violence has had an impact on Turkey’s tourism industry, a key sector of the national economy.

The ‘rain from hell’

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.

“No matter what happens, then you have to address the solution in Syria,” Francona says. “Syria is where the nexus of every problem there is in the Middle East comes to roost,” Francona said.

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