The Trump administration on Monday abruptly closed down a government program created by the Biden administration to allow migrants to use an app to secure an appointment for admission into the United States through legal ports of entry, signaling the start of President Trump’s promised crackdown at the southern border.

Moments after Mr. Trump took the oath of office, an announcement posted on the CBP One program’s website declared that the app would no longer function and that “existing appointments have been canceled.”

The program, which debuted in early 2023, allowed 1,450 migrants a day to schedule a time to present themselves at a port of entry and seek asylum through U.S. immigration courts. More than 900,000 migrants entered the country using the app from its launch in the beginning of 2023 to the end of 2024.

A former Department of Homeland Security official who spoke on the condition of anonymity said that around 30,000 migrants had appointments to enter the United States through the app as of Monday morning.

At the El Chaparral border crossing in Tijuana, Mexico, dozens of migrants who stared at their phone screens trying to check whether their appointments were still valid instead found the crushing message that they no longer existed.

“I am in shock,” said Maura Hernandez, who received the news on Monday morning as she arrived in Tijuana with her four small children from the state of Michoacán. She had a scheduled appointment on Tuesday.

“I don’t know what is going to happen to us,” she said, adding that they had fled their home amid rampant insecurity.

The program was a key part of the Biden administration’s effort to gain control over migration through the southern border. On the one hand, the administration blocked asylum for migrants who crossed illegally. At the same time, U.S. officials believed that by offering migrants an organized way to enter legally through an app, they could discourage attempts to gain entry without authorization. Border numbers have dropped dramatically in recent months, and officials believe the program is a major reason.

“I would say that the model that we have built of restricting asylum at our southern border and building accessible, lawful, safe and orderly pathways for individuals to seek relief under our laws is the model that should be sustained,” said Department of Homeland Security Secretary Alejandro Mayorkas in an interview with NPR this month. “And we have delivered the border and those accessible pathways to the incoming administration.”

The end of the program will test that theory as the Trump administration moves toward a more restrictive policy at the border. The former homeland security official said that they estimated that, in total, nearly 300,000 migrants were in Mexico waiting to use the app.

“We are so disappointed,” said Gustavo Selva from Venezuela after reading the update on his phone that the program had been shut down. He had received hopeful news of his scheduled appointment 21 days ago.

Two days ago, however, he received an email informing him that it had been delayed until Feb. 9. By then, he had already traveled to Tijuana from the southern state of Chiapas after waiting there for seven months for his appointment to go through.

“We thought we could enter today without a problem,” Mr. Selva added. “Now we will be stranded here indefinitely.”

Critics of the program, especially Republican lawmakers, viewed it as a way to allow those who otherwise had no way of entering the U.S. to come into the country and remain for years as their immigration cases languished in the courts.

“The fact that this application exists is the most underreported scandal of the Biden admin. They made an application to facilitate illegal immigration. It boggles the mind,” Vice President JD Vance said in a social media post last week.

Matthew Hudak, a former senior Border Patrol official, said the decision was a clear sign that things were changing at the southern border.

“Simply wanting to immigrate to the U.S. and signing up to get in line will be replaced by more stringent policies that will significantly raise the bar for those seeking to come here, including reimplementing the Remain in Mexico program,” he said. “Many will be left to decide if they will work through the legal process or attempt to enter the country illegally and face what will likely be much more significant consequences.”

Aline Corpus contributed reporting from Tijuana, Mexico.