An explosion ripped through the heart of Istanbul on Tuesday morning — marking a potentially major escalation of the violence plaguing Turkey.
Turkey arrested five people in connection to the bombing that left 10 people dead and at least 15 wounded.
The German Foreign Office said Wednesday that all 10 victims were German citizens.
The suicide bomber who died in the attack was identified as ISIS militant Nabil Fadil, a Saudi Arabia-born Syrian citizen who entered Turkey on Jan. 5 and registered as a refugee, the New York Times reported.
Fadil's movements were not monitored as he was not on a government watch list of terrorism suspects, Turkey's Prime Minister Ahmet Davutoglu said. He said that ISIS was behind the attack.
The Turkish interior minister on Wednesday announced the arrest of one person in connection with the bombing. Later the same day, Davutoglu announced that Turkey had detained four more suspects, the Associated Press reported.
Davutoglu said Thursday that Turkey has attacked ISIS positions in Iraq and Syria in retaliation for Tuesday's bombing, the AP reported.
Turkish artillery carried out the attacks over the last 48 hours, targeting 500 ISIS positions and killing 200 members of the extremist group, Davutoglu said.
The Turkish state-affiliated Andalou Agency reported that Davutoglu had called German Chancellor Angela Merkel Tuesday to offer his condolences after it was reported many of the victims were German.
The 10 Germans killed were part of a tour group organized in Berlin, The Times reported. Seven others in the group were also injured in the attack.
German Foreign Minister Frank-Walter Steinmeier condemned the "barbaric and cowardly act" saying, "we can't exclude that Germans are among the dead."
Merkel said that Istanbul attack showed the "need to act determined" against terrorism.
"Today, Istanbul was hit; Paris has been hit, Tunisia has been hit, Ankara has been hit before," Merkel said. "Once again, international terrorism has shown its cruel and inhumane face."
In a statement, National Security Council spokesperson Ned Price said that the U.S. strongly condemned the terrorist attack.
"This heinous attack occurred in Istanbul's historic heart, and struck Turks and foreign tourists alike," Price said. "We stand together with Turkey, a NATO ally, a strong partner, and a valued member of the Counter-ISIL coalition, in the face of this attack and pledge our ongoing cooperation and support in the fight against terrorism."
The State Department also condemned the attack in a statement and reaffirmed their "strong commitment" to working with Turkey to combat terrorism.
The blast hit the city's Sultanahmet district — home to the famed Hagia Sophia and Blue Mosque, and the first stop for many tourists in Turkey.
The attack could prove to be a bracing blow to the country, and potentially its tourism economy. More than 12 million people visited Istanbul last year.
German's foreign ministry advised travelers to avoid crowded public places and tourist attractions.
Amid the spiral of civil war in neighboring Syria and a new surge in fighting in southern Turkey between the military and Kurdish insurgents, the country has descended deeper into violence. Kurdish militants, the left-wing terror group DHKP-C, and ISIS have all carried out attacks in Turkey over the last year.
But none have targeted sites frequented by foreigners — and an attack of such magnitude in a central and well-known tourist area of Istanbul could be a major blow for the country.
Internationally, news of the blast around Turkey's beloved tourist attractions has been resonating. "I was there a month ago," wrote one woman on Twitter, posting a photo of the Hagia Sophia with its minarets rising into a clear blue sky. "We kept remarking about how we felt so safe."