U.S. Customs and Border Protection has reportedly accused New York City Mayor Bill de Blasio (D) of crossing the U.S.-Mexico border illegally.

The Associated Press reported on Wednesday that the agency made the allegations in a letter obtained by the news service. It claims that de Blasio and his security detail violated Mexican and U.S. immigration laws by crossing the border on foot while the mayor made a visit near El Paso, Texas.

The AP reports that de Blasio's office has denied the charge.

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"The mayor crossed the border with the direct approval and under the supervision of the border patrol supervisor at this port of entry,” de Blasio spokesman Eric Phillips said in an email Tuesday night to the AP. “Any suggestion otherwise is a flat-out lie and an obvious attempt by someone to attack the Mayor because of his advocacy for families being ripped apart at the border by the Trump Administration.”

The allegation appears to stem from a visit de Blasio made with about 20 other mayors to the Mexican border in Texas on June 21, which was the day after President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE signed an executive order to stop the practice of separating families who were caught crossing the southern border illegally.

De Blasio went to a holding facility for migrant children but had been denied entry, according to the AP. The news service reports that de Blasio and his security outfit, run by the New York Police Department, then went to Mexico before crossing into the U.S. to try and get a view of the facility.

The letter alleges that a Border Patrol agent witnessed a group on the Rio Grande River flood plain south of the Tornillo, Texas, port of entry taking pictures of a facility. The agent then asked them if they had been authorized to view the facility from that location.

An NYPD inspector reportedly responded that they had not and that they reached the location by coming from Mexico.

The AP adds that the agent then told the group that they had crossed the border illegally and that they'd need to be taken to an official crossing for an inspection. But the group disregarded that order and drove back to Mexico, AP reports.

The news comes as many Democratic lawmakers pressure the Trump administration about its immigration policies and how it intends to reunite the thousands of families that were separated at the border. De Blasio has also been among a growing list of lawmakers to call for the abolishment of Immigration and Customs Enforcement.