A German author and vocal critic of America’s National Security Agency was denied entry to the U.S. on Monday.

Ilija Trojanov planned on taking American Airlines from the Brazilian city of Salvador to Denver to attend a conference with German academics, but was not permitted to board the plane, reported The Local, an English-language German newspaper.

Trojanov, who writes for the German newspaper Frankfurter Allgemeine, said that he was not provided with an explanation when he was barred from taking the flight to Colorado.

“The airline worker had hardly entered my name when she paused, stood up and disappeared without explanation behind a door,” he wrote in the Allgemeine.

“A few moments later she returned with clearly a high ranking person who told me in rapid Portuguese and then almost as quickly in English that because of ‘Border Crossing Security’ she was duty bound to inform American authorities immediately about my arrival at the airport,” explained Trokanov.

The author believes that he is on America’s no-fly list because of his outspoken criticism of the NSA’s pervasive surveillance programs.

Earlier this year, Trojanov helped write an open letter to German Chancellor Angela Merkel, criticizing a large spying operation in Germany that the NSA scandal revealed. The letter called the surveillance program an “historic attack on our democratic, constitutional state.”

He used his experience at the airport to further derail the NSA and similar spying operations.

Trokanov said, “One of the most important and most threatening aspects of the NSA scandal is the secretiveness of the system. Transparency is apparently the greatest enemy of anyone who allegedly defends freedom.”

“It is more than ironic when an author who has used his voice for years to stand up to the danger of surveillance and of a secret state within a state is rejected entry into the ‘land of the brave and free’,” he added.

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