Amnesty International says that Turkish authorities have detained its country director for Turkey, Idil Eser, amid a sweeping crackdown on dissent.
The U.K.-based rights group says Eser was taken into custody Wednesday morning with seven other human rights workers and two foreign trainers.
It happened as they took part in a "routine training event" in a hotel on an island near Istanbul, Amnesty adds.
On Thursday, the rights groups said that it has learned the detainees are being investigated for alleged membership in an armed terrorist organization.
"The absurdity of these accusations against Idil Eser and the nine others cannot disguise the very grave nature of this attack on some of the most prominent civil society organizations in Turkey," Amnesty's Secretary General Salil Shetty said in a statement.
"Their spurious detention while attending a routine workshop was bad enough: that they are now being investigated for membership of an armed terrorist organization beggars belief," he adds.
NPR's Peter Kenyon in Istanbul reports that "there was no explanation for the detentions, and no immediate comment from police."
Amnesty said the detainees were denied access to lawyers for at least 28 hours, which it has said runs counter to Turkish law.
Last month, Turkish authorities detained Amnesty's Turkey country chair Taner Kiliç, accusing him of ties to U.S.-based cleric Fethullah Gulen, who they blame for the failed coup attempt last July. Kiliç "remains in jail pending trial," Reuters reports.
Amnesty says it is the first time in its history that a country has detained its chair and director at the same time.
Tens of thousands have been swept up in the purge since the coup attempt. "Turkey has jailed more than 50,000 people pending trial and suspended or dismissed some 150,000, including soldiers, police, teachers and public servants, over alleged links with terrorist groups," the wire service reports.
The Turkish government defends the measures as necessary for its security; human rights advocates say authorities are using security as a justification to clamp down on all forms of dissent.
As The Two-Way has reported, "a referendum in April, which passed by a narrow margin, radically expanded the powers of Turkish President Recep Tayyip Erdogan."
"If anyone was still in doubt of the endgame of Turkey's post-coup crackdown, they should not be now," Shetty said. "There is to be no civil society, no criticism and no accountability in Erdoğan's Turkey."