Renowned Turkish peace activist Osman Kavala was detained Wednesday by counterterrorist police at Atatürk Airport in Istanbul. Kavala is the chairman of Anadolu Kültür (Anatolian Culture), a nonprofit that fights for artistic rights and cultural diversity. He was returning on a flight from Gaziantep, in Turkey’s Anatolia region, where he was discussing a project at the Goethe Institut, when he was arrested for unknown reasons.

“Osman Kavala has worked tirelessly to build reconciliation, dialogue and support the rule of law in Turkey. Release him from detention,” pleaded Emma Sinclair-Webb, Turkish director of the Human Rights Watch, in a tweet on Thursday.

US State Department spokeswoman Heather Nauert said in an interview with Reuters that his arrest was an example of a “very alarming trend” in Turkey. Since last year’s failed military coup, more than fifty thousand people, including academics, activists, civil society leaders, human rights defenders, and journalists, have been arrested and tens of thousands have been removed from government jobs.

“We have expressed to the Turkish government our concerns on many occasions about this trend . . . It remains a major concern of ours,” Nauert told reporters in Washington, DC.

Kavala is also the founder of the International Peace and Reconciliation Initiative (IPRI), which launched in 2012 to facilitate a diplomatic resolution with the Kurdish in Turkey.

“France, like other European countries, regularly cooperates with Mr. Kavala, who is a regular interlocutor to our embassy. [We] will be very attentive to developments in this case,” said French foreign ministry spokesperson Agnes Romatet-Espagne.