The suicide bombing on Sultanahmet Square on Tuesday killed 10 people -- all of them Germans, the German Foreign Ministry confirmed Wednesday.
It was the deadliest attack on Germans abroad in more than 13 years. And Turkey's Prime Minister promised a determined effort to repel the threat.
But he added that Istanbul had "become a city of hope today in the rings of fire in the region, to the people of the Middle East, Balkans and Caucasus."
As if to underscore the government's resolve, Turkey detained 68 suspected terrorists in sweeps across seven provinces, Anadolu Agency reported Wednesday.
That included three Russians who were staying at a house in Antalya, according to an account also reported by Russia's state-run Sputnik news.
Another 21 people held in Sanliurfa were "preparing for attacks in Turkey," according to Anadolu Agency. And 16 people -- 15 of them Syrian -- were detained in Ankara for allegedly starting to scout out buildings there.
One of those caught in the security sweep is being held in connection with the Istanbul blast, according to Turkish Interior Minister Efkan Ala.
"The examination and investigation continues in multiple ways and in a very serious manner," Ala said.
Another Turkish official, speaking to CNN on condition of anonymity, said Turkey -- and the rest of the world -- would never be safe until the situation in Syria is resolved.
"As long as there is a training ground for ISIS on the other side of our border we will continue to have this problem, not only Turkey but Europe and U.S.," the official said. "Turkey remains committed to its calls for an ISIL-free zone, a region free of terrorism across its borders." ISIL is another acronym for ISIS.
The official said Turkey should try to "drive ISIS into the desert. This isn't a result of Turkish foreign policy; it's about what's happening in Syria."
Few details on the bomber
Officials quickly blamed ISIS for the attack.
Seventeen people were wounded; 11 were still hospitalized at midday Wednesday, Turkey's interior minister said.
Nine of those still in the hospital were German, one was Norwegian and the other was from Peru, according to Ala.
Few details have been released about the attacker. He was born in 1988, officials said. He came to Turkey from Syria
, registering as a refugee. He was not being tracked by Turkish security.
Ala told journalists Wednesday that authorities had not considered the suspected bomber a security risk, according to Anadolu Agency.
He said the suspect had visited the Migration Administration in Zeytinburnu, where he was registered and fingerprinted.
Asked whether there had been an "intelligence weakness," Ala replied, "Your assessment that he was fingerprinted and registered one week ago is correct. However, in terms of our current assessments, he was not among the people being searched or part of the target group, neither was he part of the target persons sent to us by other countries."
Davutoglu, the prime minister, confirmed that the suspect was not being followed. But he said the bomber was quickly identified after the attack because of the ways in which Turkey registers the entry of every foreigner into the country. He also said four other people had been detained.
His assessment of who was to blame, however, was more mysterious.
"We have determined through the attacker's entry to Turkey and his contacts that there may be active actors behind the scenes," he said.
"An ISIS connection has been identified but ISIS is a pawn, an intermediary organization, a subcontractor. We are working to reveal the real actors behind this terror organization."
Targeting Turkey and the world
Street vendors and shopkeepers opened for business Wednesday but had few customers.
A makeshift memorial appeared at the site of the attack. People laid red roses in the shadow of the city's world-famous Blue Mosque.
The suicide bomber detonated his explosives in the midst of a German tour group made up mostly of retirees, German officials said.
"We have a free society ... but there are people who want us harmed," German Chancellor Angela Merkel said. "We will persevere."
Still, the attack shocked the nation.
The headline on the German tabloid Bild asked: "Was this a targeted attack on us?"
But German Interior Minister Thomas de Maiziere said there were no indications Germans were singled out.
He encouraged Germans to continue to travel to Turkey. "We don't want to change our behavior," he said.
The last time Germany experienced such a deadly attack on its citizens was 2002. A suicide attack on a historic synagogue in Djerba, Tunisia, killed 14 German tourists, three Tunisians and two French citizens.
Some 4 million Germans visited Turkey in 2015, more than from any other country, according to the Ministry of Culture and Tourism
. Turkey is second only to Spain as a stop for German vacationers.
Upping the ante
The attack took place in an area heavily guarded by Turkish security forces.
Sajjan Gohel, international security director at the Asia-Pacific Foundation
, thought it was significant that it targeted a square that is both a draw for tourists and significant to Turkey's history and its diverse cultural identity -- the type of place, he said, "that ISIS is so deeply opposed to."
"The type of monuments that are in Sultanahmet Square are the type that ISIS has been blowing up in Syria," Gohel told CNN. "It's seen as a place where you have a mesh of different entities. It's a real melting pot."
If Tuesday's blast is confirmed to be the work of the terrorist group, that will force Ankara to step up its anti-ISIS fight, Gohel said.
"An attack like this is designed to create economic, political and social consequences," he said. "Turkey has to realize that the pipeline that feeds ISIS from Turkey to Syria has to now be cut off, because incidents like this are not one-offs. This could be part of a series of plots."
To find the likely source of the violence, one only has to look south to Syria, where a civil war
has raged for nearly five years.
The conflict has claimed more than 250,000 lives, according to the United Nations. More than half the country's 17 million residents have fled, and a humanitarian crisis remains for those left behind.
The violence can be blamed on many groups, including forces loyal to Syrian President Bashar al-Assad
. But ISIS has been behind many of the worst atrocities.
After initially doing little to fight ISIS, Turkey, a member of NATO
, has increasingly become engaged in the fight. It has begun allowing the United States to launch strikes
from Incirlik Air Base in southern Turkey, and it is trying, too, to prevent more fighters from going through its territory to join ISIS.
ISIS has responded by singling out Turkey as a primary target, and a recent issue of its Dabiq magazine had a cover showing Turkish President Recep Tayyip Erdogan alongside U.S. President Barack Obama.
Turkey's military cooperation with the United States and other NATO nations in particular has angered ISIS, said Fadi Hakura, associate fellow at Chatham House, a London-based think tank.
Investigators blamed ISIS
for two suicide blasts in October
that hit a lunchtime peace rally in Ankara, in which demonstrators were calling for an end to the renewed conflict between the Kurdistan Workers' Party, known as the PKK, and the Turkish government. Those explosions killed as many as 100 people and injured more than 240 others.