De Blasio was one of 20 US mayors who visited Tornillo's facility near El Paso, Texas, in June to better understand the conditions under which migrant children are exposed after their separation from their parents.
De Blasio says he and his team, which also includes a security detail from the New York City Police Department, had had repeated access to the facility so they wanted to get a better overview from Mexico.
"The border guards consulted with their supervisor, and they agreed, and they had our cars cross the American border into Mexico at a normal checkpoint, and we were told what the border was while we were there, and we respected them "he said at an independent press conference Wednesday in New York. He added that they showed passports in a port of entry to gain legal access.
The comments came in response to an Associated Press letter from the Customs and Border Protection to the New York Police Department, claiming that de Blasio violated the immigration laws of Mexico and the US when crossing the border illegally.
The letter states that de Blasio's group ignored CBP orders at the border, according to The Associated Press.
A senior NYPD source confirmed to CNN that the department had received a letter. The letter did not demand arrest, but called the department's attention, the source said.
CNN has not seen the contents of the letter and contacted CBP for comment.
The mayor called the letter a "distraction" and said, "You can see if our government is doing something that hurts people, they do not want that to be the focus of attention, so try to get the attention now they try to divert attention from a policy that is literally called family separation. "