Turkish police detained 21 suspects in raids across four provinces centered in Istanbul as part of an investigation into the outlawed Revolutionary People’s Liberation Party-Front (DHKP-C), authorities announced Tuesday.

The Istanbul Chief Public Prosecutor’s Office said the operation targeted suspects alleged to be active in the group’s "Grup Yorum,” "Prisoners' Families Solidarity Association (TAYAD)” and "Mahalle Alan” networks.

Police carried out simultaneous raids at 23 addresses in Istanbul, Hatay, Ankara and Kocaeli, detaining 19 suspects in the initial operation, the statement said. Authorities also seized numerous organizational documents and digital materials during searches.

Separately, police searched the Idil Cultural Center in Istanbul’s Şişli district, which prosecutors described as a gathering place for people acting in line with the group’s ideology. Two more people were detained there and added to the investigation.

Efforts to capture four additional suspects for whom detention orders were issued were continuing, prosecutors said.

Türkiye, the United States and the EU classify DHKP-C as a terrorist group.

The terrorist group pursues a far-left ideology and has been actively carrying out attacks and assassinations in the country since the 1980s, but its campaign of violence hit a snag when faced with Türkiye’s barrage of counterterrorism operations.

The DHKP-C’s most high-profile attacks include a suicide bombing that targeted the U.S. Embassy in the capital Ankara in 2013 and the killing of a prosecutor in an Istanbul courthouse in 2015.