MCALLEN, Texas -- The Trump administration is strengthening a key immigration program in two critical areas along the southern border – as officials say they must keep up with shifting migrant flows that are adapting to U.S. efforts.

The Migrant Protection Protocols (MPP), commonly known as the “Remain-in-Mexico” policy, was established in January and expanded over the summer amid greater cooperation with Mexico. The policy involves returning migrants to Mexico as they await their immigration hearings, with courtrooms set up at key border sites, instead of them being released into the interior.

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Beginning Friday, the agency is further expanding MPP to the Tucson Sector of Arizona. It comes in response to a surge in migrant flows in that area, reflecting groups adapting to the implementation of MPP at other hotspots along the border.

Officials say that migrants crossing the border there will now be bussed over to El Paso, where they can be returned to Mexico and there is capacity and shelter -- and MPP has already been in place for six months.

“We track pretty specifically the different flows and where folks are crossing the border, and what we saw as we turned on MPP across the border, the flows stopped in those locations and they went to a different location,” Acting DHS Secretary Chad Wolf told Fox News. “So this continues to be a dynamic process, and it’s not ‘we put one thing in place and it solves the problem.' Every day and every week we’re having to adjust.”

The strengthening at Tucson was first reported by The Washington Post, which reported that officials estimate DHS will send at least one busload a day to El Paso.

In addition to Tucson, the agency is planning on strengthening MPP at Del Rio Sector as well in the coming days and weeks, Fox News has learned.

Officials will use similar measures as in Tucson but also intend to make this a location where migrants can be returned to Mexico as well, and then return to Laredo, Texas for court proceedings. But this requires cooperation with Mexico, as well as additional preparations across multiple agencies that can take weeks.

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“We’re strengthening our MPP program in Tucson sector and Del Rio sector because what we’ve seen as we’ve tracked these flows is they’re going in these sectors where MPP is maybe not as strong as it could be,” Wolf told Fox News. “So we’re continuing to reassess that and putting in and strengthening those measures, and we’ll see them move elsewhere but that’s part of what Border Patrol does, what CBP does and what DHS does.”

MPP has been a cornerstone of the administration’s immigration policy, specifically in its quest to end “catch-and-release” where migrants are released into the U.S. to await hearings. The administration flagged that practice as a pull factor that was drawing migrants north at a rate that marked a new migrant crisis for the region and for the U.S.

But, as the Mexican government increased cooperation with the U.S. after President Trump threatened to slap tariffs on its southern neighbor, it has coincided with dramatic results at the border – including a 70 percent decrease in apprehensions since the highs of May. It has been combined with a series of regional agreements on migration – including one with Guatemala, which saw migrants starting being flown there on Thursday.

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While humanitarian groups have expressed concerns about MPP, warning the migrants could face violence on return to Mexico, the Trump administration says it drastically cuts down the time it takes to adjudicate a claim from years to just a few months.

However, the program faces a significant headwind in the form of a court challenge that could wreak havoc on those efforts. The liberal Ninth Circuit Court of Appeals is expected to rule on the case in the coming weeks.