As it turns out, desperate refugees, human smugglers and drug traffickers aren't the only people who illegally cross the US-Mexico Border.

To that list, we can add New York City Mayor Bill de Blasio and his security entourage. According to a letter sent to the Associated Press, de Blasio has been accused of violating both US and Mexican laws after he crossed the border on foot during a recent visit to an immigration detention center near El Paso, Texas. The mayor responded to the accusations on Wednesday, calling them "absolutely ridiculous."

De Blasio has been intensely critical of the Trump administration's immigration policies and has declared New York City a sanctuary for illegal immigrants.

The incident occurred when de Blasio visited the Texas border in a group with 20 other mayors on June 21, the day after President Trump signed an order stopping family separations at the border. The mayors were going to document what they described as a "humanitarian crisis" created by the president. After being denied entry to a holding facility for immigrant children separated from their parents, de Blasio then went to Mexico to get a better view of the facility with his security detail. The NYPD is in charge of de Blasio’s security.

While de Blasio's group was standing in the Rio Grande flood plain taking photos of the holding facility, a uniformed Border Patrol agent noticed the group and approached them. The agent asked if anyone from Border Patrol or public affairs had accompanied them to authorize their visit. A member of de Blasio's security detail said no, and when the agent asked how they got there, they gestured toward Mexico.

The agent then informed them that they’d crossed the border illegally and asked them to stay put while he informed his supervisor and took them to an official crossing for an inspection per federal law. But the group ignored the agent and walked back to their vehicles and proceeded to drive back into Mexico. They later re-entered by car through a port of entry about three hours later, the letter said.

In response to the letter, de Blasio declared the accusation "absolutely ridiculous" and said he had received approval to cross from a border patrol supervisor, according to the Daily Caller.

The mayor’s spokesman Eric Phillips maintains the group did nothing wrong and was authorized to be there.

"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 in an email to the Associated Press. "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."

De Blasio told the AP that he and the members of his detail showed their passports and crossed with approval of agents at the entry point.

"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.

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

De Blasio characterized the letter as a threat organized by the Trump Administration to silence him.