Security sources said on Thursday that F.O.M., one of the leaders of the terrorist group Revolutionary People’s Liberation Party-Front (DHKP-C), was detained in a joint operation by Turkish intelligence and counterterrorism police in Istanbul.

Media outlets reported that the operation took place on Wednesday evening as F.O.M. arrived aboard a flight from an unspecified country. Some media reports said he came from Switzerland, though authorities did not confirm this.

F.O.M. was sentenced to 10 years and five months in absentia on three separate charges in an earlier trial and had an arrest warrant. He is also the brother of O.M., a terrorist killed in a shootout with Turkish security forces in 2016 in eastern Türkiye.

The group is listed as a terrorist organization by Türkiye, the U.S. and the European Union.

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. In February, two members of the terrorist group were killed when they opened fire on police officers outside a courthouse complex in Istanbul as they tried to storm the place.