FILE - In this Thursday, June 21, 2018, file photo, New York Mayor Bill de Blasio speaks alongside a group of other U.S. mayors during a news conference outside a holding facility for immigrant children in Tornillo, Texas, near the Mexico border. U.S. Customs and Border Protection is alleging that 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. (AP Photo/Andres Leighton, File)

FILE - In this Thursday, June 21, 2018, file photo, New York Mayor Bill de Blasio speaks alongside a group of other U.S. mayors during a news conference outside a holding facility for immigrant children in Tornillo, Texas, near the Mexico border. U.S. Customs and Border Protection is alleging that 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. (AP Photo/Andres Leighton, File)

WASHINGTON (AP) — New York City Mayor Bill de Blasio and his security detail violated both Mexican and U.S. immigration laws by crossing the border on foot during a visit near El Paso, Texas, U.S. Customs and Border Protection alleges in a letter obtained by The Associated Press.

The mayor on Wednesday called the allegation “absolutely ridiculous.”

De Blasio, a fierce critic of the Trump administration’s immigration policies, went to the Texas border with about 20 other mayors from around the country on June 21, the day after President Donald Trump signed an order stopping family separations at the border.

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More than 2,300 families were separated as a result of the administration’s zero tolerance policy, which criminally prosecutes anyone caught crossing illegally. The mayors said Trump had failed to address a humanitarian crisis of his own making.

De Blasio went to a holding facility for immigrant children but was denied entry. He then went to Mexico to get a better view of the facility. The New York Police Department runs de Blasio’s security detail.

According to the letter, a uniformed Border Patrol agent noticed a group standing in the Rio Grande flood plain south of the Tornillo, Texas, port of entry, taking photos of the holding facility. The agent asked if anyone from Border Patrol or public affairs was there to authorize their presence. An NYPD inspector said no, according to the letter, and when the agent asked the group how they arrived, they pointed to Mexico.

The agent told them they’d crossed the border illegally and asked them to remain there while he got a supervisor and took them to an official crossing for an inspection per federal law, according to the letter. But the group disregarded the order, walking back to their vehicles and driving back to Mexico, according to the letter. They re-entered by car through a port of entry about three hours later, the letter said.

De Blasio said border agents approved the crossing after his security detail asked to venture into Mexico so the mayor and other elected officials could get a different view of facility holding youths.

De Blasio said the agents checked with their supervisor before allowing the officials’ cars to exit and return via normal checkpoint.

“While we were there, we were told where the border line was and we respected it,” said de Blasio.

He said both times they showed their passports and crossed with approval of agents at the entry point.

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“Threats by the Trump Administration will not stop me from speaking out and they won’t stop my fellow mayors from speaking out, and they won’t stop every day New Yorkers and Americans from speaking out,” de Blasio said.

He said it was an attempt to distract from the government’s “inhumane policy.”

“When our federal government is doing something that’s hurting people, they don’t want that to be the focus of attention. So now, trying to get the focus off a policy literally called ‘family separation,’ they’re trying again to distract.”

A spokesman for the Border Protection had no comment.

“U.S. Customs and Border Protection and the United States Border Patrol take violations of law very seriously,” the letter said.

The letter was sent June 25 by Aaron Hull, the chief patrol agent for the Border Patrol’s El Paso sector, to New York Police Department Commissioner James O’Neill.

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Associated Press Writer Michael R. Sisak contributed to this report.