Turkish prosecutors issued detention warrants Tuesday for 254 Istanbul municipality and ministry staff over alleged links to the Gülenist Terror Group (FETÖ) which is accused of orchestrating last year's attempted coup.

The Chief Public Prosecutor's Office in Istanbul issued arrest warrants for 112 municipality employees of the city on Tuesday as part of a probe into the FETÖ.

The suspects are former and active employees of the Istanbul Metropolitan Municipality as well as of 18 district municipalities across the city, a judicial source, who spoke on condition of anonymity due to restrictions on talking to the media, said.

They are believed to have used the ByLock mobile app that was allegedly used by FETÖ members during last year's deadly defeated coup.

The ByLock smartphone app is believed to have been cracked by Turkish security agencies, allowing them to identify tens of thousands of suspected FETÖ supporters.

So far, at least 67 suspects have been detained during the raids that followed the issuance of the warrants. Operations remain ongoing to apprehend the remaining suspects.

The Chief Public Prosecutor's Office in Ankara issued arrest warrants for 121 National Education Ministry personnel and 21 former Ministry of Youth and Sports staff over the use of Bylock application. In a statement, the prosecutor's office said the Ankara-based probe was being conducted throughout eight provinces' public institutions and organizations.

A total of 116 out of 121 Ministry of National Education staff had already been dismissed for using the ByLock app, said the statement.

The Interior Ministry said on Monday nearly 1,000 people were detained over the previous week on allegations of ties to FETÖ.

Turkey accuses FETÖ and its U.S.-based leader Fetullah Gülen of having orchestrated the defeated coup of July 15, 2016, which left 250 people dead and nearly 2,200 injured.

Since then more than 50,000 people have been jailed pending trial over links to FETÖ, while 150,000 people have been sacked or suspended from jobs in the public and private sectors.

Ankara also accuses FETÖ of being behind a long-running campaign to overthrow the state through the infiltration of Turkish institutions, particularly the military, police and judiciary.

Gülen has lived in self-imposed exile in the U.S. state of Pennsylvania since 1999.