The number of migrants who crossed the U.S.-Mexico border without authorization in June plummeted to the lowest level since the start of the Biden administration following the enactment of stricter asylum rules, according to unpublished government data obtained by CBS News.

Border Patrol agents recorded just over 100,000 apprehensions last month of migrants who entered the U.S. illegally along the southern border, a sharp drop from the 169,000 apprehensions reported in May, the preliminary statistics show.

Border Patrol apprehensions denote the number of times the agency processed migrants who crossed into the U.S. in between legal ports of entry, which is illegal. They do not include migrants processed at ports of entry, where the Biden administration has been admitting tens of thousands of asylum-seekers each month.

The last time Border Patrol apprehensions along the U.S.-Mexico border were lower was in February 2021, President Biden's first full month in the White House. The number of unlawful border entries remains high compared to pre-pandemic levels.

Still, the marked reduction in illegal crossings has, at least temporarily, eased the major operational, humanitarian and political challenges faced by the Biden administration over the past year as a result of an unprecedented migration crisis that saw record numbers of migrants arrive to the U.S.-Mexico border.

In May, daily illegal border crossings peaked at 10,000, a record, before officials terminated the Title 42 pandemic measure that allowed them to expel many migrants on public health grounds, without allowing them to claim asylum. In June, average daily migrant apprehensions dropped below 4,000.

While a single catalyst is unlikely, U.S. officials and immigration experts said, the drop in illegal crossings could stem from stricter asylum rules enacted by the Biden administration in May, programs that allow some would-be migrants to enter the country legally, expanded efforts by Mexico and other Latin American countries to slow U.S.-bound migration and tougher rhetoric by American authorities. Moreover, temperatures in the southern U.S. have soared to dangerous levels this summer, contributing to several deaths of migrants in recent weeks.

The U.S. has also increased regular deportations, which impose stiffer penalties, such as five-year banishments and the threat of criminal prosecution, since halting the Title 42 expulsions. While Title 42 allowed officials to summarily expel hundreds of thousands of migrants, it did not impose these penalties, and fueled a massive jump in repeat crossings among those expelled to Mexico.

"Some of that drop can be attributed to the strengthened consequences that we are implementing at the border," Blas Nuñez Neto, the assistant secretary for immigration and border policy at the Department of Homeland Security, told CBS News in an interview Wednesday.

Nuñez Neto said the administration is staging the "most significant expansion of the use of expedited removal in DHS history," describing a process dating back to the 1990s that allows U.S. border officials to deport migrants without court hearings if they don't ask for refuge or if they fail their initial asylum screenings.

That expansion of expedited deportations has been facilitated by a Biden administration rule that disqualifies migrants from asylum if they enter the U.S. illegally without first seeking protection in another country. Nuñez Neto said the measure has deterred migration by reducing the percentage of migrants who pass their initial asylum interviews from the pre-pandemic average of over 80% to below 50%.

"What we've seen in the past is that, because of the congressional inaction and inability to address the underlying factors in our immigration system that are contributing to these now-regular surges in migration under presidents of both parties, migrants are coming to the border to claim asylum because they know that the system is broken and it will take years for them to go through the process," Nuñez Neto added.

Nuñez Neto also credited the Biden administration's efforts to increase opportunities for migrants to enter the country legally for the decrease in illegal entries. A phone app known as CBP One is allowing up to 44,950 asylum-seekers in Mexico to enter the U.S. each month at ports of entry, while another program is giving 30,000 Cubans, Haitians, Nicaraguans and Venezuelans the chance to fly to the U.S. monthly.

Other countries south of the U.S. have simultaneously staged operations to crack down on migration. 

"You're seeing Mexico taking actions on its southern border to disincentivize migrants from entering Mexico. You're seeing Guatemala do the same. Colombia and Panama are currently doing an operation, a coordinated operation, in the Darién (jungle) that is unprecedented in its scope," Nuñez Neto said.

The recent reduction in illegal immigration numbers, however, has not extinguished the intense Republican criticism of the Biden administration's border strategy. Republicans have accused the administration of engaging in a "shell game," saying apprehensions are not an accurate measure of progress at the southern border since they do not include those who evade capture and migrants who enter the U.S. via the Biden administration's legal migration programs.

"This is another way of hiding the ball or cooking the books to make it look like the situation at the border has vastly improved when it has not," Texas Republican Sen. John Cornyn recently said in Congress. 

Angela Kelley, a top Biden administration immigration official until her departure in May 2022, said the CBP One system "is bringing order to the border," noting that those entering the U.S. under that process are doing so without the help of smugglers and after being screened by law enforcement officers.

"A 'shell game' implies you're moving things around in order to hide them. Whereas what we're seeing now at the border is we know where people are and they're not hiding. They're coming forward," she added.

The new strategy has also not pleased advocates for migrants and human rights groups, who have argued the Biden administration is relying on restrictive Trump-era policies to deter migrants from coming to the U.S.

Robyn Barnard, an attorney at Human Rights First, a group that advocates for migrants and refugees, said it is "perverse" for the Biden administration to cite a restriction on asylum as a reason for lower border crossings.

"For an administration who says they are pro-immigrant and wanting to welcome asylum-seekers, for them to be touting this ban as a success is very disappointing, because it clearly contravenes our obligations to refugees," Barnard said, referring to the restriction on asylum eligibility.

While migration to the U.S. border remains significantly lower than the record levels recorded over the past year, the Biden administration's new border strategy could be upended by lawsuits, seasonal changes to migration patterns and the continued movement among people fleeing poverty and political upheaval across Latin America, including millions of migrants displaced from crisis-stricken Venezuela.

The regulation that restricts asylum eligibility has been challenged by immigration advocates who say it is a draconian policy that flies in the face of U.S. refugee law and by Republican-led states who argue it contains too many exemptions. Republican state officials have also asked a judge to shut down the program that allows certain migrants to fly to the U.S. legally if they have American sponsors. 

Nuñez Neto, the DHS border official, said the administration still expects to see "a lot of migration in coming weeks and months."

In:
• Immigration

Camilo Montoya-Galvez

Camilo Montoya-Galvez is the immigration reporter at CBS News. Based in Washington, he covers immigration policy and politics.

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