De Blasio, who has taken time away from his duties as mayor to protest the Trump administration's immigration policies, went to the Texas border with almost 20 other mayors from around the country on June 21, the day after President Donald Trump signed an order halting family separations at the border. The agent then asked them if they had been authorized to view the facility from that location.
The New York Police Department provides the security detail for de Blasio.
The agent reportedly asked the group if anyone from Border Patrol was there to authorise the visit and asked how they had arrived.
The agent then claims that when they approached the group and asked if they were authorized to be in the area, a New York Police Department inspector had said no. But the group disregarded the order and drove back to Mexico, according to the letter.More news: Kylian Mbappe revels in England's World Cup misery on Instagram
"The mayor crossed the border with the direct approval and under the supervision of the border patrol supervisor at this port of entry", Phillips said.
De Blasio said the agents checked with their supervisor before allowing the officials' cars to exit and return via normal checkpoint.
"We fully expect to not be told the truth, we fully expect to be turned away the way senators and congressmen have been turned away just trying to get the honest truth about what's happening to these children just as all of you have been doing", de Blasio said at a press conference at the time.
Mayor Bill de Blasio stops at a gate after being told he could not cross through the gate to the tent facility setup at the Tornillo-Guadalupe Port of Entry on June 21, 2018 in Fabens, Texas.More news: US President Donald Trump arrives in United Kingdom for four-day visit
The government had been bound by court order to reunite children aged five and under by 10 July, but United States officials say that "legitimate, logistical impediments" are delaying reunions for numerous 102 children under five years old.
"When our federal government is doing something that's hurting people, they don't want that to be the focus of attention".
KVUE has reached out to the U.S. Customs and Border Protection for a statement.
The letter, addressed to New York Police Department Commissioner James O'Neill, said a uniformed border official noticed the group taking photos of the holding facility from the Rio Grande River flood plain - south of the United States point of entry in Tornillo, Texas.More news: Firefighter killed in Wisconsin explosion
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