One of the things that most shocks visitors to smuggling routes along the U.S.-Mexico border is the proliferation of IDs.

Passports, identity cards and other documents are strewed on the ground, cast aside by illegal immigrants who are erasing their old lives as they slip across the border to try to capitalize on more relaxed immigration policies to gain a foothold in the U.S.

On a recent investigative trip to Jacumba, California, two human trafficking experts watched it happen in real time.

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A smuggler, wearing a balaclava to conceal his face, was helping adult illegal immigrants climb a rope ladder over the fence and then handing a child through the bars. The smuggler snapped proof-of-life photos — a way of showing whoever was paying the migrants’ smuggling fees that they had made it into the U.S. — and then the Border Patrol arrived.

“They basically just in front of everybody are smuggling these people through. In 30 seconds, Border Patrol picks up the people, puts them in their truck, which they call the ‘Migrant Uber’ down there,” said Jarrod Sadulski, a human trafficking expert. “They open up the door, the people know to get right in. They know that this is their claim to asylum, and they’re here.”

Mr. Sadulski and Ali Hopper, his coresearcher, said they found the formalization and normalization of illegal border crossings to be shocking.

“It is ridiculous what’s going on down there, and it’s on a daily basis,” he said.

Ms. Hopper added: “It basically is an assembly line.”

In that analogy, the smuggling cartels are running the plant, the migrants are the widgets coming off the line, and the U.S. Border Patrol is somewhere between quality control and FedEx, doing the final processing and delivery.

“Our Border Patrol is their transit. We transfer and transport the products for them — the products being the children, the women, the men, the labor, the sex, the organs,” Ms. Hopper said. “There’s no incentive for them to change what they’re doing because the process works for them.”

The formalization of illegal immigration has helped usher in record numbers of illegal immigrants over the past three years.

Illegal crossings at the southern border fell in recent months. President Biden credits his harsh rhetoric and changes to shift migrant traffic away from the border and to airports, where some unauthorized migrants can enter as long as they schedule their arrivals.

That has reduced some of the pressure on the Border Patrol.

For those still making the crossing, however, things can go very wrong. In Jacumba, the researchers said they saw a van full of women’s underwear.

“That’s where they’re being raped,” Mr. Sadulski said.

The researchers also found discarded documents describing when to resist arrest, which they said were carried by migrants making the crossing.

Cartels run the identity-erasing operation, allowing migrants to whitewash their histories and start new lives unconnected to any past criminal issues or other red flags.

The researchers said cartel operatives are set up near the border, in stash houses or even mobile locations such as minivans, and print out fake documents for the migrants before they cross.

Ms. Hopper said people from heavily scrutinized countries, such as Iran or Afghanistan, may shed their identities multiple times, including when they first arrive in the hemisphere and then again when they reach the U.S. border.

Whatever they cross with is how the Border Patrol enters them into U.S. databases.

“If somebody comes in and says their name is Mickey Mouse, they’re getting entered into the system as Mickey Mouse,” Mr. Sadulski said. “The cartels realize that by creating these false identities, it really facilitates the entrances into the United States a lot smoother.”

Ms. Hopper said that’s a problem for U.S. agencies that have to deal with the migrants later and don’t know who they are.

“It’s a mentality of trash in, trash out,” she said. “They’re getting identities and background information from these migrants, but already when they’re getting the information, it’s already fake.”