U.S. immigration officials have sent 240 asylum seekers back to Mexico to await their court hearings as part of the Trump administration's new "Migrant Protection Protocol" initiative. Department of Homeland Security officials shared the update on a call with reporters on Tuesday morning.

So far, all of the immigrants subject to the new program — which many refer to as "Return to Mexico" — have entered the United States through the San Ysidro port of entry, which connects San Diego to Tijuana. In the "coming weeks," DHS plans to expand the program to other places along the southern border, an official said.

Here's how the program works, according to an official on the call: A limited number of migrants are legally allowed to enter the United States through San Ysidro. From there, immigration officials "collect biographical" information, "do an interview and take a sworn statement." Migrants wait in holding facilities while officials check for contraband, perform a "basic medical assessment," and determine "which language each person is comfortable having a dialogue in." After that, officials determine whether the immigrant is "potentially amenable to the MPP," or, in other words, whether the immigrant can wait in Mexico for the next steps in their asylum process.

If an asylum seeker qualifies for the program, agents return them to Mexico with a court date, instructions on "when and where to appear," as well as a list of "free or low cost legal providers." Those instructions are given in English and Spanish.

"Return to Mexico" is one the biggest changes to immigration policy implemented by the Trump administration. When it was announced in December, it immediately drew criticism from immigration advocates who called it a "due process disaster." Immigration advocates have also been concerned about the safety of migrants in Tijuana, the dangerous Mexican border city where shelters are at capacity and cartel violence is rampant.

A group of immigration advocates — including the American Civil Liberties Union and the Southern Poverty Law Center — officially challenged the policy in February, filing a lawsuit against Homeland Security Secretary Kirstjen Nielsen and her department. The groups claim that the program is in violation of of the Immigration and Nationality Act, the Administrative Procedures Act and various international human rights laws.

The new policy was meant to address concerns that asylum seekers fail to show up for court hearings, according to Nielsen.

"Aliens trying to game the system to get into our country illegally will no longer be able to disappear into the United States, where many skip their court dates," wrote Nielsen in December.

However, Department of Justice data shows that 89 percent of asylum seekers were present for their court hearings in fiscal year 2017. About 93,000 migrants claimed asylum in the United States in fiscal year 2018.