The explosion, which happened in Sultanahmet Square, not far from the Hagia Sophia and the famous Blue Mosque that are two of the city’s most famous buildings, comes after a surge in violence elsewhere in the country over the last 12 months as a fragile peace agreement with Kurdish rebels has unraveled.

Recept Tayyip Erdogan, the country’s controversial president, said the attack appears to have been carried out by a suicide bomber “of Syrian origin.” There was, however, no initial confirmation of this. Erdogan has cracked down heavily on independent media over the last year, and Turkey’s government immediately imposed a temporary ban on coverage of the incident, the website Today’s Zaman reported.

Deputy Prime Minister Nuyman Kurtulmus said most of the dead Tuesday were foreign tourists, and German Chancellor Angela Merkel said Germans were “probably” among the victims, according to Der Spiegel. By contrast, when a bomb killed 97 people in Ankara in October, the victims were overwhelmingly Kurdish opponents of the government attending a peaceful demonstration. The people behind that blast haven’t been identified.

If Erdogan’s thesis is correct (Kurtulmus said the perpetrator had been identified from body parts), it will be the most shocking evidence yet of how far the Syrian civil war has destabilized neighboring Turkey, raising the risk of dragging the region’s largest country even deeper into the multi-dimensional conflict. However, officials were also pursuing another, no less worrying, line of inquiry.

The Wall Street Journal cited a Turkish official as saying “we’re considering two options: ISIS or PKK.”

The revolutionary PKK, also known as the Kurdish Workers’ Party, abandoned a two year-old ceasefire with Ankara last year after the Turkish government bombed PKK bases in Syria. The resumption of an armed struggle for a Kurdish state undermined the more moderate Kurdish Democratic Party’s (HDP) efforts to improve the lot of Turkey’s 11 million Kurds through the political process. The HDP subsequently lost ground in parliamentary elections as Erdogan’s AK party tightened its grip on power.

The so-called Islamic State is an altogether newer problem for Turkey, and one with which it has had an ambivalent relationship in the last 18 months. Russia’s Vladimir Putin has accused it of helping to finance the self-styled Caliphate with purchases of smuggled oil–claims that Ankara denies. But Turkey has done little over the last few years to stop jihadis from Europe and Africa traveling across its territory to join IS, and its troops stood by last year while IS overran the Kurdish-held town of Kobani on the Syrian border with Turkey.

Erdogan is also backing other rebels against the government of President Bashar al-Assad. It was, ostensibly, to protect these that its aircraft shot down a Russian warplane at the end of last year, causing Russia to retaliate with severe economic and diplomatic sanctions.

Turkey’s currency sold off briefly to near a new all-time low against the dollar hours after the attack, but the selling reversed by early afternoon. The country’s main stock index also recovered from an initial drop of 0.8% to be up on the day.