The Associated Press reports: Germany’s Foreign Ministry says it has contacted U.S. authorities over their decision to deny a German author entry to the United States.

Ilija Trojanow says he was refused permission to board a flight from Salvador da Bahia, Brazil, to Miami on Monday without explanation. He told the Spiegel Online website that the denial of entry might be linked to his criticism of the U.S. National Security Agency.

Trojanow was one of several prominent German authors who signed an open letter to Chancellor Angela Merkel urging her to take a firm stance against the mass online surveillance allegedly conducted by the NSA.