Erdogan did not specify which group, specifically, his government thinks is responsible for Tuesday's explosion, which happened between the Hagia Sofia and Blue Mosque tourist attractions in the cultural and historic heart of the city. And no group immediately claimed responsibility.
Yet it's no secret ISIS and various militant factions have been battling in Syria, with Turkey siding against those groups.
At least 15 people were wounded in the explosion around 10:20 a.m. (3:20 a.m. ET), the Istanbul governor's office said. The city's Sultanahmet Square, already a heavily guarded area, was swarming with security forces and ambulances in its aftermath.
"Investigations continue about the explosion's cause, the explosive's types, the perpetrator or perpetrators of the event," said Istanbul governor Vasip Sahin. Read more on edition.cnn.