Early Saturday, the Istanbul Chief Public Prosecutor’s Office issued 55 arrest warrants for suspects accused of being part of the FETO network and of financing the group. 

Among those arrested were businessmen and air traffic controllers. An operation to apprehend the remaining 20 suspects is still underway, according to the source who spoke on the condition of anonymity.

In a separate operation to Land Forces Command across Turkey, 16 of 77 suspects, who were issued arrest warrants earlier, were also arrested on Saturday, police said. 

The suspects are reportedly among the so-called "imams" of the group, the source added. 

Led by U.S.-based Fetullah Gulen, FETO is accused of orchestrating the July 15 coup plot in Turkey as well as being behind a long-running campaign to overthrow the state through the infiltration of Turkish institutions, particularly the military, police and judiciary.