The bombs hit a district popular with evening strollers in Istanbul
Turkish police have reportedly detained nine people over the bombings in the city of Istanbul on Sunday that killed 17 people and injured at least 150.
The suspects allegedly have links to the bombers, the Anatolia news agency reports without giving details.
No group has claimed responsibility but security services say the attacks bear the hallmarks of Kurdish separatists.
However, the PKK separatist group has denied any role in the attack, the bloodiest in Turkey for several years.
Turkish Prime Minister Recep Tayyip Erdogan earlier implied the PKK was to blame, saying the bombings were a "cost" of the government's military campaign against the PKK.
The PKK has bombed Turkish civilians before but so have Islamist militants and radical leftists.
PKK leader Zubeyir Aydar said earlier his group was not linked to the attacks.
"We think this attack was carried out by dark forces," he added, in what appeared to be a reference to ultra-nationalists.
Sunday's first bomb was planted in a rubbish bin in the Gungoren area. It caused crowds to gather before a second, larger device detonated.
The blasts occurred about 10 minutes apart - the first at around 2200 local time (1900 GMT) on a busy pedestrian street.
There were scenes of panic, with people covered in blood as they tried to run from the scene. TV footage showed many victims lying on the street and being carried to ambulances in blankets.
Five children were among the dead.