REUTERS/Murad Sezer
A woman takes photographs in front of the New Mosque by the Bosphorus strait in Istanbul, Turkey January 12, 2016.
And ISIS' new focus could lead to more attacks in Turkey's popular tourist areas and undermine US efforts to help secure a porous border that the jihadists frequently use to their advantage.
ISIS (also known as the Islamic State, ISIL, or Daesh) has not claimed responsibility for the attack, but the Turkish government said the bomber was linked to the group.
Four people have been detained in connection with the bombing, Turkish officials said Wednesday. ISIS has been known not to claim attacks inside Turkey, possibly to agitate ethnic tensions as various groups in the country blame one another.
What makes this attack different from others that have occurred in Turkey over the past year is that this one targeted tourists. Nine of those killed in the Istanbul bombing were German nationals, according to Turkish officials.
Previous terrorist attacks inside Turkey have been aimed at local political actors. An October attack, for example, targeted a pro-Kurdish peace rally in the Turkish capital of Ankara, and other attacks have targeted Kurdish or leftist national groups. Turkish police and Kurdish militias have been clashing in southeastern Turkey, and Turkish government officials have been ramping up rhetoric denouncing ethnic Kurds who want an autonomous region that stretches along the shared borders of Turkey, Iraq, and Syria.
"This is the first [attack] that's not aimed at a group linked to the national Kurdish movement," Aaron Stein, a senior fellow at the Atlantic Council, told Business Insider. "It's the first one that is uniquely aimed at foreign tourists inside of Istanbul."
"Clearly this is an attack inspired by or at least similar to some of the attacks you've seen in Tunisia and to a far lesser extent the one you saw in Paris," he added, referring to the November Paris attack that left 130 people dead.
Ths Istanbul bombing represents a "clear change in tactics [for ISIS] inside Turkey," according to Stein.
"They're attacking Turkish interests and those linked to the state rather than Kurdish nationalism that would operate underneath the state," Stein said.
"That is the clear difference here. One outcome of the attack is that tourists who may have been going to Turkey … may cancel their trips now because the balance inside Turkey has moved to this urban center of Istanbul directly."
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Police forensic officers near the scene of the explosion in Istanbul.
There could be several possible motives for targeting tourists, experts say. Damaging Turkey's economy is just one.
"By striking in the heart of Istanbul's old city, which has many mosques, museums and tourists, but few Turks, ISIS is targeting Turkey's lucrative tourism industry, which brings the country more than $30 billion in revenues," Soner Cagaptay, director of the Turkish Research Program at The Washington Institute for Near East Policy, told Business Insider in an email.
And though the nationality of the victims of this latest attack might be a coincidence, Stein pointed out that Germans constitute the largest number of foreign tourists who visit Turkey each year.
"I definitely think that you'll see an effect on the Turkish economy," Stein said. "... Tourism is a massive industry inside of Turkey, and any sort of dip will affect their economy, and we're already dealing with a decrease in Russian tourists in Turkey."
Indeed, some German tourists are already returning home after the attack, and the German government has warned tourists in Turkey to stay away from crowded open areas.
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And attacking Germans could serve dual purposes for ISIS. The bombing hits at Turkey's largest tourist segment as well as German policy.
"ISIS seems intent on creating an anti-refugee backlash in Europe, hoping to fuel already rising anti-Islam sentiments on the continent," Cagaptay said.
The bombing could "drive further backlash against German Chancellor Merkel's pro-Syrian refugee policy," Cagaptay said.
This argument is strengthened by Turkey's admission on Wednesday that the suicide bomber who was allegedly responsible for the attack entered the country as a refugee and wasn't on a terrorism watch list. The Turkish government also said that, though the bomber is thought to have been an ISIS member, the terrorist group is being used as a "subcontractor" by "secret actors."
Another motive for ISIS could be centered on the US and Vice President Joe Biden's upcoming planned visit to the country.
"By bringing the war to Istanbul, Turkey's largest city, ISIS also aims to preemptively undercut Turkish-US cooperation against that group, which was meant to start in a matter of weeks following Biden's visit to Turkey on January 23," Cagaptay said.
"The Turkish-US plan is to take a 60-mile-long territory held by ISIS across the Turkish border in Syria, plugging the last gap for ISIS access to Turkey and Europe from Syria."
The oft-porous border between Turkey and Syria has served as a major transit point for both foreign fighters traveling to Syria to join ISIS and fighters who are already in ISIS territory and want to get into Europe to stage attacks.
Cagaptay said that ISIS "knows that this move will cripple its finances and recruitment drive and prestige and is, therefore, acting preemptively to hurt Turkey so Ankara will stand down or at least cool off against the group."
Strategic security firm The Soufan Group predicted that more of these types of attacks inside Turkey will likely occur.
"The suicide bombing in the heart of Istanbul's still thriving tourist industry ... is an all-too-replicable attack," the firm wrote in a note on Wednesday. "Such attacks do not require much planning, communication, or skill - outside of assembling the explosive vest."