U.S. Customs and Border Protection is alleging that New York Mayor Bill de Blasio illegally crossed from Mexico into the U.S. while visiting the El Paso, Texas, area in June, an accusation the mayor's office flatly denies.

In a letter obtained by The Associated Press, Customs and Border Protection says de Blasio and his security detail were on the Rio Grande River flood plain, south of the Tornillo Port of Entry, during a June 21 trip to the border. The letter says a Border Patrol agent asked why they were there and told them to stay put but that the mayor's entourage instead drove back to Mexico.

De Blasio's spokesman says the security detail had the direct approval and was under the supervision of the Border Patrol supervisor.