HOUSTON — The U.S. government on Friday expanded its requirement that asylum seekers wait outside the country to a part of the Texas Rio Grande Valley across from one of Mexico’s most dangerous cities.

The Department of Homeland Security said that it would implement its Migrant Protection Protocols in Brownsville, Texas, across the border from Matamoros, Mexico. Homeland Security says it anticipates the first asylum seekers will be sent back to Mexico starting Friday.

Under the so-called “Remain in Mexico” policy, asylum seekers are briefly processed and given a date to return for an immigration court hearing before being sent back across the southern border. Since January, the policy has been implemented at several border cities, including San Diego and El Paso, Texas. At least 18,000 migrants have been sent back to Mexico under the policy, according to Mexico’s National Migration Institute.

The U.S. is trying to curtail the large flow of Central American migrants passing through Mexico to seek asylum under American law. The busiest corridor for unauthorized border crossings is the Rio Grande Valley, at Texas’ southernmost point.

The policy announcement came as groups of lawmakers visited the region Friday to examine detention facilities operated by the U.S. Border Patrol, including the processing center in McAllen, Texas, where hundreds of adults and children are detained in fenced-in pens.

Standing outside the processing center, Sen. Jeff Merkley, D-Ore., criticized conditions inside the facilities and other Trump administration programs cracking down on asylum seekers.

“We want them treated with dignity and respect as we would want our family members to be treated,” he said.

Homeland Security said it had coordinated with the Mexican government to expand the policy. But the Trump administration has pressured Mexico to crack down on migrants, threatening earlier this year to impose tariffs until both sides agreed on new measures targeting migration.

Matamoros is at the eastern edge of the U.S.-Mexico border in Tamaulipas state, where organized crime gangs are dominant and the U.S. government warns citizens not to visit due to violence and kidnappings.

The city is also near where a Salvadoran father and his 23-month-old daughter were found drowned in the Rio Grande, in photos that were shared around the world.

Many people, including families with young children, have slept for the last several months in a makeshift camp near one of the international bridges. Thousands more stay in hotels, shelters or boarding houses. Only a few migrants daily have been allowed to seek asylum under another Trump administration policy limiting asylum processing known as “metering.”

A list run by Mexican officials has more than 1,000 people on it, said Elisa Filippone, a U.S.-based volunteer who visits Matamoros several times a week to deliver food and donated clothes. But many others not on the list wait in shelters. There are rumors that migrants are shaken down for bribes to join the list, Filippone said.

She described a desperate situation that could be made worse if people are forced to wait longer in Mexico for their asylum claims to be processed.

“I’m afraid that Matamoros is about to catch on fire,” she said.

DHS recently implemented the “Remain” policy for migrants in Nuevo Laredo, across from Laredo, Texas. About 1,800 asylum seekers and migrants are currently waiting in Nuevo Laredo, where some have reported being kidnapped and extorted by gangs.

“I don’t want to go out on the street. I’m afraid the same men will do something to me or my boys,” said one woman, insisting on speaking anonymously out of fear for their safety.

People in Nuevo Laredo were told to return in September for U.S. court dates. At other points of the border, wait times have stretched to several months.

Unlike in criminal court, the U.S. government does not have to provide lawyers to people in the immigration court system. Attorneys in South Texas have long questioned where they could meet with potential clients in Tamaulipas.