Though an investigation is already underway, ByLock was first heard of publicly after last year's bloody coup attempt. The messaging app was exclusively used by the Gülenist Terror Group (FETÖ) long before the putsch bid blamed on its infiltrators in the army was held, investigators say. Teams of IT experts now work to decipher 1 million messages between members of the terrorist group.

They managed to decode 18 million messages so far. However, the remaining1 million messages reportedly contain "critical" orders or instructions to the group's followers by FETÖ's senior figures.

ByLock was discovered during criminal inquiries into the terrorist group whose criminal activities have been under the spotlight since two coup attempts in 2013. The National Intelligence Organization (MİT) uncovered the messaging app apparently programmed - or modified for exclusive use of the group's members - by someone linked to FETÖ. According to recent media reports, police intelligence staff linked to the terrorist group were behind the app. The FETÖ-linked staff working in a powerful intelligence department of the Turkish National Police were the "architects" of the app, or rather its modification to serve the purposes of the group. A group of intelligence officers are accused of controlling the private app used to deliver FETÖ leader Fetullah Gülen's messages to his followers, as well as to instruct the group's members on how to carry out plots against anti-Gülenists.

Investigations show that 95 out of the first 100 people who downloaded and installed the app were personnel of police intelligence and the other five people were employees of the Scientific and Technological Research Council of Turkey (TÜBİTAK). TÜBİTAK was the target of mass infiltration by Gülenists in the past, and it is believed that the original developers of the application are linked to this state-run institution.

The ByLock investigation was expanded after the coup attempt and thousands of people accused of using the messaging app for communicating Gülen's messages to subordinates and for pro-terrorism propaganda have been detained or arrested.

Servers of the app deployed in Lithuania were brought to Turkey where teams from the intelligence service work to decode.

Turkish authorities so far detected some 102,000 people who downloaded the app and used it for correspondence over the group's activities. Prosecutors launched investigations and thousands, ranging from shopkeepers to high-ranking generals and bureaucrats, housewives and prominent businesspeople, were detained for exchanging messages via ByLock for acts of terror.

Most of the defendants claim they "accidentally" downloaded the app and never used it while others claim they did not use it for FETÖ messages. However, the messages, including those urging FETÖ members to help the coup plotters who killed 249 people in last year's coup, point out that the app was one of the most employed means of communication in the secretive group. Other encrypted messaging apps would be used by FETÖ members after the authorities discovered ByLock's use.

Ankara Chief Prosecutor's Office and the MİT are coordinating the investigation into the encrypted messages that will help investigations into FETÖ as well as finding the group's members disguising themselves.

The terrorist group, which posed as the religious "Hizmet" (Service) Movement for decades, had moved to seize power in multiple coup attempts over the past four years when the government moved to curb its widespread clout in Turkey. Since then, a string of investigations disclosed that Gülenists were involved in a wide array of crimes, ranging from money laundering to orchestrating sham trials to imprison critics and conspiring against anyone opposing FETÖ.