You mean INA Sec. 212(a)(3)(A)(i), for terrorist-related activity?
"Any alien who a consular officer or the Attorney General knows, or has reasonable ground to believe, seeks to enter the United States to engage solely, principally, or incidentally in- (i) any activity (I) to violate any law of the United States relating to espionage or sabotage or (II) to violate or evade any law prohibiting the export from the United States of goods, technology, or sensitive information... is inadmissible."

