Police conducted an Istanbul-based operation against the Gülenist Terror Group (FETÖ) Wednesday leading to the detention of 21 suspects. Four others with outstanding arrest warrants remain at large.

Istanbul's organized crime and intelligence units raided several locations in the city as the culmination of an investigation. The investigation focused on so-called "gaybubet (absence) houses" of the terrorist group its members use as safe houses. These houses are used by wanted FETÖ members who often use forged IDs and rarely step out to avoid capture. Among those captured and wanted were suspects who were earlier dismissed from their jobs in the public and private sectors on the suspicion of affiliation with FETÖ.

Since December 2013, when the terrorist group emerged as the perpetrator of two coup attempts disguised as graft probes, FETÖ has been regarded as a security threat. Prosecutors say that the group's infiltrators in law enforcement, the judiciary, bureaucracy and the military had waged a long-running campaign to topple the government. The group is also implicated in a string of cases related to its alleged plots to imprison its critics, money laundering, fraud and forgery.

As its activities face heightened scrutiny following multiple attempts to seize power, FETÖ apparently strove to hide its fugitive followers, according to its former followers. A former member who testified to prosecutors has said that the number of the group's "gaybubet houses" increased from 75 to 560 across Türkiye. Authorities believe that number might be even higher.

FETÖ has been under more intense scrutiny since the July 15, 2016 coup attempt its infiltrators in the army carried out. The Ministry of National Defense announced last year that 24,387 members of the Turkish Armed Forces (TSK) were sacked since the coup attempt for possible ties to the group, while administrative inquiries are underway for 781 others.