An American academic has been arrested in Thailand on accusations of insulting the monarchy, prompting the United States late Tuesday to express alarm and to urge authorities to respect freedom of expression.

A man with Dallas ties is fighting to free his brother from a Thai prison after he was arrested for a crime they say he didn’t commit. Last week, the Thai military served American scholar Paul Chambers with an arrest warrant, alleging that he insulted the country’s monarchy.

The U.S. State Department said it was alarmed at the arrest of Paul Chambers, a lecturer in civil-military relations, under Thailand’s strict lèse-majesté laws.