At least 23 people are now known to have died in the attacks, which have been linked to al-Qaeda.
More than 300 people were injured and about 70 of them remain in hospital.
Six of the dead were identified as Jews, one of them an eight-year-old girl, with most of the other victims Muslim passers-by and at least one policeman.
Turkish and Israeli experts are working together on the investigation, but th Arab newspaper Al-Qods Al-Arabi says it has received a claim of responsibility from al-Qaeda.
Hundreds of Turks have left flowers outside the synagogues wrecked in the bombings – it is believed as many as 400 kilograms of homemade explosives were used.
Israel’s foreign minister Silvan Shalom has visited the site, while Turkey’s Prime Minister Recip Tayyip Erdogan has vowed to leave no stone unturned in the hunt for justice.
As a sign of the close ties between the Jewish-state and Turkey, Israeli secret agents have been assisting investigations, combing the city looking for clues to the identity of the bombers.
“I am sure the Turkish government will do everything to find out who is behind the attacks and bring them to trial in a short time,” Israeli Foreign Minister Silvan Shalom told reporters in Istanbul, where he laid wreathes to honour the dead.
The bombings were the latest in a series of strikes against Jewish targets, including suicide attacks in the Moroccan city of Casablanca in May that killed 45 and an attack on an Israeli-owned hotel in Kenya a year ago that left 18 dead.
NATO member Turkey has been Israel’s chief regional ally since 1996 when they struck a military cooperation accord, much to the anger of Arab countries and Iran.