Turkish officials said on Saturday that Usama bin Ladin's al-Qaida network may have had a hand in the car-bomb blasts that struck the synagogues as Jewish worshippers celebrated the Sabbath.
"The Turkish probe has found that the two separate attacks were suicide bombings," a diplomatic source close to the investigation told Reuters.
Nine Jews were killed in the attacks. The other victims included Muslim passers-by outside the houses of worship. At least 242 people were wounded in the bombings.
Israeli investigators are also reportedly investigating Saturday's attacks.
Police have detained three people, including two women, in connection with the attacks, private television channel NTV said in a report officials have not confirmed.
A Turkey-based Islamist group, the Islamic Great Eastern Raiders/Front (IBDA/C) claimed responsibility to the state-run Anatolia news agency.
The double bombing turned the
streets into war zones
But Turkish officials said the scale of the attacks showed that foreign groups played a role in the operation.
The local press, also pointed the finger of blame at al-Qaida.
"Ladin's terror has hit us as well," the popular daily Vatan said. "Istanbul's two most prominent synagogues were hit in quick succession just as the twin towers (in New York) and the US embassies in Kenya and Tanzania."
"This is dastardly," Aksam newspaper said. "The Muslims' holy month and the Jews' sacred day is soaked in blood."
At least six Jews are among the dead, Israeli and Jewish officials said, with Israeli consul Amira Arnon saying the attacks could have been the work of a "major organisation, maybe with a state behind it".
The bombings were the latest in a series of strikes against Jewish targets in recent months, including 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.
"The Turkish probe has found that the two separate attacks were suicide bombings"
War zone streets
The attacks left a trail of carnage and destruction that turned streets into war zones, with twisted wreckage of burnt-out vehicles, collapsed buildings and smashed out windows.
Police set up barricades to block off the streets as investigators in bright yellow helmets combed through the carpet of broken glass and debris in the hunt for clues to find out who was behind the attacks.
US President George Bush denounced the bombings "in the strongest possible terms" while UN chief Kofi Annan said he was "appalled" by the attacks.
Turkey has been Israel's chief regional ally since 1996 when the two nations struck a military cooperation accord, much to the anger of Arab countries and Iran.
It was the first Muslim country to recognise Israel after the creation of the Jewish state in 1948.