Amid ongoing investigation into users of the encrypted communication application ByLock, 125 personnel of the Istanbul Police Department were detained early Tuesday.

The sweeping arrests included 30 deputy police chiefs who are accused of using the amateur ByLock app to send encrypted messages to fellow members of the Gülenist Terror Group (FETÖ). Members of the terror group used the smartphone application to share information and communicate with each other anonymously.

Police have been investigating the users of ByLock since the deadly coup attempt on July 15. Members of FETÖ allegedly began using another app called Eagle after Turkish intelligence cracked the ByLock app. Prime Minister Binali Yıldırım spoke about these two communication networks in the first days of August, saying that FETÖ members used these two encrypted apps for communicating infiltration plans for state institutions. He added that some 50,000 members have used the ByLock and Eagle system.

In addition to the ongoing investigation, Turkey's National Intelligence Organization (MİT) is also trying to decode messages sent through the applications. Prosecutors in the case say ByLock was popular among Gülenists as a form of communication between 2013 and 2015, noting that after 2015 the terror cult turned to Eagle IM, which offers "256-bit, end-to-end AES encryption" for its users, according to its description on the Google Play Store. The app's developers also promise protection from attacks by the "man in the middle" or hackers who intercept chat threads between users and alter the messages of one of the two parties. Similarly, ByLock offered an eight-digit password for users and highly-secure encryption.

Investigations reveal that both those in the lower ranks of FETÖ's system within state institutions and those in the upper echelons of the bureaucracy made use of ByLock. Testimony by Sinan Çörekçi revealed that a senior Gülenist in the northwestern city of Kırklareli installed the app on the phones of those with intimate ties to FETÖ. Çörekçi, a former court clerk in the city, was detained after the coup attempt and told prosecutors that Hakan Altun, a Gülenist figure responsible for organizing infiltration within the local judiciary, told him and others to communicate through ByLock.

The same app was found on the cellphone of Dinçer Ural, a brigadier general who acted as a legal adviser to the Chief of the General Staff. Ural was detained for his links to the terror group and had made contact with Dilaver Azim, a prominent Gülenist figure who according to prosecutors was entrusted with controlling FETÖ funds. The Doğan News Agency reported that Ural and Azim regularly exchanged messages via ByLock between 2011 and 2015. A report in the Habertürk daily says the MİT decoded some 18 million encrypted messages of more than 50,000 people.