AQTAU, Kazakhstan -- French journalist Vincent Prado has been detained in Kazakhstan while investigating the 2011 deadly protests in the southwestern town of Zhanaozen.

Prado's interpreter, Danara Ismetova, told RFE/RL by phone that police on September 27 detained her and Prado in Aqtau, the capital of the Manghystau region, and took them to a local police station.

A representative of the Kazakh migration police, Zhandos Qalmyrzaev, confirmed to RFE/RL that Prado and Ismetova were in custody but declined to give any details as to why they had been detained.

Reporters Without Borders (RSF) called on the Kazakh authorities to immediately release Prado, saying that "his detention constitutes an unjustified obstruction of press freedom."

According to the RSF, Prado has Kazakh press accreditation.

Kazakh police shot dead at least 16 people during a crackdown on an oil workers' protest in Zhanaozen in December 2011.

International and domestic human rights groups condemned the repression against the protesters, who were demanding payment of overdue salaries.

Dozens of activists, several police, and local officials were sentenced to prison terms afterward.

Most of Kazakhstan's leading independent and opposition media outlets were banned following the crackdown.

Kazakhstan is ranked 158th out of 180 countries in RSF’s 2018 World Press Freedom Index.