15 senior military officials were detained over their ties to the Gülenist Terror Group (FETÖ) Friday during simultaneous operations in nine Turkish provinces.

According to a statement by the office of the Ankara chief prosecutor, seven of the detained officials were colonels and nine were lieutenant colonels, adding that 12 were still on active duty.

FETÖ's infiltration into the military has been discussed since the 1990s. Authorities revealed that more than 2,000 military students were forced out of military schools over the past decade.

FETÖ, led by U.S.-based Fetullah Gülen, is accused of orchestrating the foiled coup attempt that killed 250 people on July 15, 2016. Through its members in the military, ranging from low-ranking officers to generals, the terrorist group sought to seize power and assassinate President Recep Tayyip Erdoğan, according to prosecutors.