DOUALA, Cameroon — An advocacy group says a Cameroonian-American writer who criticized the government over months of Anglophone protests has been detained in the Central African nation.

Patrice Nganang had written an article for Jeune Afrique in which he was critical of how Cameroon has handled a sometimes violent secessionist movement in some English-speaking areas.

PEN America says in a statement that Nganang’s detention as he tried to leave the country “demonstrates the high cost of free expression and press freedom in Cameroon.”

Nganang went missing from the airport in Douala on Thursday. He is a dual U.S. citizen and a professor at Stony Brook University in New York.

President Paul Biya, in power since 1982, has criticized what he calls secessionists among the country’s English-speaking minority, who have complained of discrimination.