TIJUANA, Mexico (AP) — In a packed camp for migrant families on Mexico's side of the busiest U.S. border crossing, Nelson Membreño has lived through a chickenpox outbreak, people's heavy drug use and night prowlers wielding knives.

But he isn't more vulnerable than the thousands of others waiting in the camp in Tijuana for their chance to apply for humanitarian protection in the United States. He was surprised to get a call that he and his family were picked to seek asylum.

“God opened the door,” the 30-year-old from Honduras said before a border officer shouted his name. Wheeling a large suitcase past concrete barriers topped with barbed wire, Membreño walked into the U.S. with his wife, son and stepson.

His confusion speaks to an opaque — if temporary — system the Biden administration has assembled that tasks immigration advocates with choosing which migrants get a limited number of slots to come to the U.S. to claim asylum.

President Joe Biden has kept in place a Trump administration order that quickly expels people from the country without a chance to seek asylum to prevent spread of the coronavirus. While Biden exempted children traveling alone shortly after taking office, his administration also is quietly allowing more families and single adults to avoid the ban. A Justice Department attorney said in federal court Tuesday that a new order dealing with children was coming this week, without elaborating.

There is neither a published list of advocacy groups deciding who is vulnerable enough to claim asylum nor an explanation of how they choose people, with migrants often learning by word of mouth. Final decisions on asylum rest with U.S. authorities, who don’t disclose their criteria or say how many people are admitted to the country.

An advocacy group used to send psychologists tent to tent in the Tijuana camp of about 2,000 migrants to identify families who were the most vulnerable. Those who qualified got numbered, laminated cards that put them in a queue to claim asylum. The coalition stopped issuing cards when it discovered profiteers were selling them for $500 to $1,000 each.

Several advocates are uneasy about their unusual assignment. Soraya Vazquez, deputy director of advocacy group Al Otro Lado’s Tijuana office, calls it “the best of bad options,” noting that her group facilitated entry for about 2,000 people as of early July.

“This way of doing things is definitely not fair," said Vazquez, whose group collected 13,000 online questionnaires by early July asking people about their migration histories, medical issues and safety threats living in Mexico.

Advocates say U.S. officials have provided some guidance to characterize cases as urgent, though the specific criteria isn't public. They include serious medical conditions, imminent physical danger, being LGBTQ or a single mother with young children. But many chosen fall outside those categories.

Recently in Tijuana, volunteer attorney Ian Seruelo of San Diego interviewed about 20 asylum-seekers staying at the camp who scored an appointment in the office of Border Line Crisis Center, part of a network of advocacy groups. The network has searched for vulnerable people at the camp but shifted to migrant shelters, hoping to dispel beliefs that the best chances of getting picked are at the increasingly dangerous and unsanitary camp.

Migrants aggressively trailed advocates who visited the camp, asking, ‘Why are you helping them and not us?’ Seruelo said. With a mix of Latino and Black migrants, accusations of racism flew. The attorney says he feels “put in a corner" choosing who gets into the U.S.

Seruelo spends about 10 minutes with each person in a cubicle, focusing his questions narrowly on living conditions in Tijuana. Their reasons for fleeing their homelands are left for a U.S. immigration judge.

Silvia Portillo, 34, sat with an infant on her lap and told him that she had a difficult pregnancy and that a knife-wielding man threw a rock at her tent in the camp, threatened to set it on fire and asked for money.

“I do not feel safe, but I'm there out of necessity,” said Portillo, who arrived from Honduras in a 2018 caravan, married a Mexican man and has lived in the camp since February because she cannot afford rent.

Manuel Antonio Segovia of El Salvador says he volunteers as a security guard at the camp, where criminals extorting migrants have punched him in the stomach and chest. Roberto Mejia of Guatemala says a man pulled a gun on him and others told him to leave the camp.

Migrants who are chosen are tested for COVID-19 and typically released into the United States, where a judge will rule on their request for asylum.

Since March, the American Civil Liberties Union has acted as an intermediary by forwarding requests from advocacy groups in Tijuana and elsewhere along the border for up to 35 families a day. The ACLU said in May that about 2,000 people were admitted through the efforts to settle a lawsuit it filed against the government. ACLU attorney Lee Gelernt said the Biden administration won’t allow a more recent number to be released.

Separately, advocacy groups also have been choosing up to 250 asylum-seekers a day in other locations, not including San Diego. The group HIAS has facilitated entry for 2,857 people as of July 2, while the International Rescue Committee said it arranged entry for 540 people.

The U.S. Homeland Security Department didn't answer specific questions but said in a statement last month that the exemptions were aimed at “identifying and lawfully processing particularly vulnerable individuals who warrant humanitarian exceptions.”

Nelson Hernandez, 33, of El Salvador, figured he may have his 4-year-old daughter or elderly mother-in-law to thank for his family getting picked from the Tijuana camp, but he's not sure.

“God wanted it,” he said before a border official called his name and he set foot on U.S. soil. The family was headed to relatives in McAllen, Texas.

For Membreño, he thinks he may have been chosen because he followed advocates for hours as they visited the camp. Within hours of crossing the border, he was released in San Diego and making plans with his family to join relatives in North Carolina.

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Associated Press writer Julie Watson in San Diego contributed to this report.