On Sunday night, hundreds of families waiting to seek asylum in the United States were jolted by the sound of gunfire near Casa del Migrante, a shelter housing them in the Mexican border city of Ciudad Juárez. “Last night we heard a bunch of automatic fire and they said they attacked a police station here. If the police aren’t safe here, then what about us?” a Honduran man staying at the shelter asked Texas Monthly this week.

The gunfire the migrants heard on Sunday appears to be the latest in a series of organized crime attacking Juárez police officers that led the U.S. consulate to issue a security alert warning people to stay away from police stations. But the Trump administration says it will soon begin forcing asylum seekers from Central America and elsewhere to stay in Juárez and other Mexican border cities—many of them besieged for years by drug-driven violence—while their cases are decided by U.S. immigration courts. It is the latest example of how the Trump administration has used El Paso, which Trump intends to visit on Monday, as an early launching point for immigration policy.

The Department of Homeland Security formally implemented what it calls “Migrant Protection Protocols” last month at the San Diego-Tijuana border. So far, the program has focused on a handful of single adults who were returned to Mexico after having their initial asylum claims processed at a U.S. port of entry. But DHS officials last week said they planned to expand the practice to families and begin implementing it at other border sites, beginning with the El Paso-Juárez border, the Washington Post reported. The administration hasn’t said when it will start enforcing the policy in El Paso-Juárez or other Texas border areas.

“We have implemented an unprecedented action that will address the urgent humanitarian and security crisis at the southern border. This humanitarian approach will help to end the exploitation of our generous immigration laws. The Migrant Protection Protocols represent a methodical commonsense approach, exercising long-standing statutory authority to help address the crisis at our southern border,” Homeland Security Secretary Kirstjen M. Nielsen said, announcing the implementation of a policy that the Trump administration had previously called “remain in Mexico.”

The Mexican government has given conflicting statements about its views on the policy, but most recently said that it won’t agree to extend the policy beyond one crossing in Tijuana. DHS says the policy could apply to most people seeking asylum, whether they made their request at a port of entry or crossed illegally between ports and surrendered to Border Patrol agents. Children who arrive without a parent or guardian at the border won’t be sent to Mexico, U.S. officials have said.

Longstanding U.S. policy has allowed asylum seekers to legally live in the United States while immigration courts decided their claims, a process that often takes years because of a backlog of more than 800,000 cases. The administration has said it expects legal challenges to its new policy, although no lawsuits have yet been filed.

If fully enforced, the new policy could result in tens of thousands of migrants waiting in Mexican border cities for their cases to be heard in U.S. courts. Since the latest surge of Central American migrant families began in October, Immigration and Customs Enforcement has released hundreds of asylum seekers per day in the Rio Grande Valley and El Paso after they showed they had a credible fear of persecution if returned to their home country. The migrants generally stay in a Texas border city for a day or two before taking a bus or flight to join family members elsewhere in the United States while their cases proceed through the courts.

But if the Migrant Protection Protocols had been enforced in the El Paso area last week, about 2,500 people could have been required to wait in Juárez until their court case came up in El Paso. Each subsequent group of arriving migrants would face the same fate; within months, tens of thousands of people could be waiting in Juárez.

Juárez’s sole major shelter for migrants, Casa del Migrante, housed almost 500 people this week and has a capacity for about 1,000. Since November, thousands of people seeking to present themselves for asylum at a U.S. port of entry—a legal process Nielsen has urged them to follow rather than crossing illegally between ports—have been forced to wait at Casa del Migrante until U.S. officials say they can process them. The migrants are each assigned a number that’s written on a wristband, and about 40 people each day have their number called, allowing them to make an asylum claim and legally enter the United States.

On Monday, hundreds of migrants took advantage of 70-degree weather to spend the afternoon in the courtyard of the shelter run by the Juárez Catholic diocese. In interviews with Texas Monthly, they said they were from Honduras, Guatemala, Venezuela, and Cuba. They spoke on condition they wouldn’t be identified by name because they fear retaliation from gangs in their home country or from officials in Mexico and the United States. Most of them said they expected their number to be chosen this week, allowing them to begin the asylum process. Unless the Trump administration moves quickly, they probably will avoid the new rules that would require them to stay in Mexico instead of the U.S. during their court cases.

The migrants said they’d feel trapped if they were required to remain in Mexico. They said it would be too dangerous to go home, and they fear exploitation or violence if they stay and wait. “It is difficult for one to work here because they don’t value you. They look at you as a migrant and they pay you whatever they want. They don’t value your work,” a man from Honduras said. A woman from Honduras, who traveled for months with her daughter to reach the border, said there’s little distinction between police and criminals in Mexico. “The federals [police] look into your clothing and your body and then they take your money,” she said.

The fear of Mexican police is a frequent concern expressed by asylum seekers at the border, said Alyssa Isidoridy, a lawyer and Masiyiwa-Bernstein fellow in the Refugee Advocacy Unit at Human Rights First, who recently visited with migrants waiting in Tijuana for the opportunity to apply for asylum. “A common refrain that I heard [from those] waiting in Tijuana was that police were sometimes the ones initiating violence against the migrants that were waiting in the city,” she said.

Isidoridy said requiring asylum seekers to wait in another country undermines due process by making it difficult—if not impossible—for them to get legal representation. “The fact of the matter is that there’s not enough legal representation for people in the U.S., and now having to create a second organized system of having lawyers being able to access people in Mexico is a huge undertaking, especially as more and more people pile up over there,” she said. That’s one of several issues that make it likely that the courts will block the policy requiring asylum seekers to remain in Mexico, Isidoridy said. “I think as we’ve seen in the asylum ban and the Muslim ban, a lot of the policies that the Trump administration is rolling out trying to prevent asylum seekers from accessing their rights here in the U.S. are not well thought out and they’re very vulnerable to legal challenges,” she said.