Just five hours after terrorists killed 42 people at an Istanbul airport, travelers returned to the site of the carnage — much to the surprise of some forensics experts.

“I find this totally astonishing,” said Professor Larry Kobilinsky of the John Jay College of Criminal Justice. “I’ve never seen such a massive crime scene looked at for five hours. It’s just impossible. You’re going to compromise, you’re going to contaminate evidence. … They should not have turned this open to the public.”

Passengers walked over shards of glass as workers tried to wash away blood. The stench of smoke still lingered in the air after three gunmen opened fire and then detonated explosives at the Ataturk Airport.

But the airport’s operator defended its actions and its security.

“The Istanbul Ataturk Airport is the last point of departure for many U.S. and European airports. Because of this reason, our security standards comply with the U.S. and EU security standards,” TAV Airports CEO Sani Sener said.

And unlike many American and European airports, “we have security checkpoints at the entrance of the terminal building,” he said.

Those extra measures didn’t stop the trio of suicide bombers with guns from terrorizing Ataturk, the 11th busiest airport in the world.

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.

“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.”

Victims from around the world

Many of the 42 victims killed were Turkish, including 10 airport staff members, Sener said.

The attack also killed six Saudis and wounded 27 more, the Saudi Arabian foreign ministry said.

The other victims 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.

Of the 239 people wounded Tuesday night, 128 remained hospitalized Wednesday, officials said.

Who’s responsible?

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.

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.

‘Bloody boot marks’

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.

“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.”

Video from inside the terminal shows the bright orange flash of fire from one of the explosions. Victims stagger. Some fall on the slippery, blood-covered floor.

Another video shows a gunman dropping his weapon 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, a bomb detonates.

The three terrorists

Of the three bombers at the airport, two were at the international terminal and the third terrorist was in a 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.

Even though Ataturk Airport is “one of the most secure airports in the world,” CNN senior law enforcement analyst Tom Fuentes said, the airport has been “very overwhelmed for several decades with terrorism from PKK.”

Experts say Turkey is especially vulnerable due to the variety of 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.

The timing

ISIS promised an uptick in attacks during the holy month of Ramadan, which is nearing its end.

The terror group has reason to detest Turkey: The country is helping the U.S.-led coalition attack ISIS targets in neighboring Iraq and Syria. Turkey allows coalition planes to fly raids from its territory.

Adding to the instability: 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 over the last few months, “the Turks have really changed their focus from only the PKK to going after ISIS as well,” Francona said.

The previous attacks

Turkey has been devastated by a string of terror attacks over the past year as it weathers bombing campaigns by both ISIS and Kurdish militants.

The Tuesday attack in Istanbul is the eighth suicide bombing in Turkey so far this year; at least 140 people have been killed.

The violence has also rattled Turkey’s tourism industry, a key sector of the national economy.

‘Like 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.”

And if ISIS is responsible for the attack, Soner Cagaptay of the Turkish Research Program at the Washington Institute said he believes that Turkey “will retaliate with full war.”

“I would expect that Turkey’s vengeance will come down like rain from hell,” he said. “For Turkey now, fighting the so-called Islamic State is going to be priority number one.”