Despite being riddled with vermin and dirt, the dark and narrow tunnel connecting Ciudad Juárez, Mexico, with El Paso, Texas, is one of the most sought-after routes for migrants to pay for a VIP “ticket” to the United States.

Those who can afford to use the tunnel will pay at least $6,000 to the cartels, according to top Mexican state officials, federal law enforcement officials on both sides of the border and migrants waiting to cross the Rio Grande who spoke to USA Today.

An immigrant smuggler named Ricardo told the facility that he charged up to $15,000 for access to this route.

In the case of VIPs, it all depends on the code provided to them by the cartels, which determines which cartel the “travel agencies” work with. These codes, often delivered via mobile phone, ensure that VIP migrants are not disturbed by local police or rival cartels during their journey.

A senior Mexican official who spoke to USA Today said an ongoing joint investigation by Mexican and U.S. authorities has found that one of the Juárez-based cartels, La Linea, is smuggling at least 1,000 migrants a month through sewage networks into El Paso.

Experts say there has been a shift in the approach of criminal syndicates, away from drug dealing and towards human trafficking.

“Criminals have moved on from their core business of drug trafficking,” Arturo Velasco, head of the anti-kidnapping unit in the Chihuahua state attorney general’s office, told USA Today. “Currently, 60-70% of their attention is focused on migrant smuggling.”

“A kilogram of cocaine can fetch $1,500, but the risk is very high,” he added. “The costs and benefits of human trafficking are $10,000, $12,000, $15,000.”

According to migrants and government officials, the VIP transit system relies on a steady stream of bribes from municipal police to high-level Mexican immigration bureaucrats.

“Corruption in Juárez, or in any other Mexican border city, must result from collusion with the authorities,” said Oscar Hagelsieb, former deputy special agent in charge of the city’s U.S. Homeland Security Investigations Unit.

Velasco told USA Today that the Mexican National Guard and immigration authorities are turning over migrants to cartels and selling migration permits that allow legal travel through Mexico.

“From inside the shelters, together with officials from the National Institute of Migration, they send information about people, and then outside, criminal groups abduct these people,” he said.

In Juárez, Velasco said, local police play a key role in migrant smuggling operations.

Asked by USA Today, Juárez Police Chief Cesar Omar Muñoz Morales denied the corruption allegations and said it was “difficult and complicated” to “deal with cases that are not formally documented.”

“It is difficult to answer your question when there are no formal complaints for our department to address,” Muñoz said. “We’re doing everything we can.”

Migrants from all over the world come to Ciudad Juárez, and some, like the Chinese, pay eye-watering fees of up to $75,000 for a VIP package, Ricardo said.

Those running out of money are camped on the banks of the Rio Grande, where they say they feel like hostages, wavering between wanting to stay away from the cartels but stay within the U.S.’s radar.

Andrés, a 25-year-old Venezuelan, told USA Today he was becoming desperate after his cartel travel agency stopped responding to him when he ran out of money.

The only option, he said, would be to jump or crawl through a concertina-wire barrier and get past Texas National Guard members before reaching U.S. immigration authorities.