At least 21 staffers of Turkey’s top broadcasting authority were detained Tuesday as part of an anti-terror investigation into the Fetullah Terrorist Organization (FETO), according to a security source.

The employees of The Radio and Television Supreme Council (RTUK), who were suspended from duty in July, were detained over their suspected links to the group, which is accused of orchestrating July 15 coup attempt, said the source, speaking on condition of anonymity due to restrictions on talking to the media.

Seventeen suspects were held in Ankara, while three were detained in Istanbul and one in central Aksaray province, the source said, adding an operation was ongoing to detain seven other suspects.

Led by U.S.-based Fetullah Gulen, FETO is accused of orchestrating Turkey’s July 15 coup plot 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.

The July 15 defeated coup left 241 people martyred and some 2,200 injured.