The source, speaking on condition of anonymity due to restrictions on talking to the media, said anti-terror police had identified the attackers as members of the United Freedom Forces, a group reportedly formed in December 2014 by Turkish far-left groups fighting in Syria.
The source said the group, based in Kobani, Syria, cooperates with the Syrian Kurdish Democratic Union Party (PYD) and Turkey’s Marxist-Leninist Communist Party (MLKP). Both groups are considered terror organizations by Turkey.
The Istanbul offices of the Yeni Safak and Yeni Akit newspapers were simultaneously attacked in the early hours of Thursday morning by masked assailants who threw Molotov cocktails and fired pump-action shotguns.
There were no casualties in either attack.
Istanbul police’s Anti-Terrorism and Intelligence Branch identified the suspects through security camera footage in the areas surrounding the newspapers’ offices and are working to identify others who took part, the source said.
The attacks were strongly condemned by President Recep Tayyip Erdogan, Prime Minister Ahmet Davutoglu, media groups and foreign ambassadors.
Later Friday, the U.S. State Department said violence against journalists is "always unacceptable" and said the U.S. defended the right of these newspapers "to print and report without fear of reprisal, which is the right of every journalist”.
"We urge Turkish authorities to investigate thoroughly this incident and to prosecute the perpetrators in accordance with Turkish law," spokesman Mark Toner said.