By Juan Montes and Alicia A. Caldwell

The surge in illegal immigration across the southern U.S. border is shaping up to be the biggest in 20 years. Unlike migrant surges in 2019 and 2014, which were predominantly made up of Central American families and unaccompanied children, so far this one is being driven by individual adults.

Most of the migrants are Mexicans, often men in search of work with the pandemic easing and the U.S. economy set to boom. Apprehensions at the southern border totaled 382,000 from the beginning of the fiscal year in October through February, up 42% compared to the same period of 2019 -- a year that saw the highest number of apprehensions since 2007. In 2020, the influx of migrants plummeted due to the pandemic.

Single adults account for 82% of the apprehensions so far this fiscal year, according to U.S. Customs and Border Protection data. Some 60% of all single adults apprehended were Mexicans. Border patrol agents say the majority of single adults they catch are men, entering to look for work such as picking fruits and vegetables, roofing and dishwashing.

The influx of children arriving alone at the border has captured broad attention. While apprehensions of mostly Central American families and unaccompanied minors have grown in the past few months, their numbers overall are still much smaller than those of adults.

The number of people traveling as families caught trying to cross the border rose to 39,000 during the first five months of this fiscal year, from just over 37,000 during the same period in 2020. During the same period in 2019, more than 136,000 people traveling as families were arrested at the border.

Some are camping out at the border in hopes that the Biden administration will soon allow them the chance to come into the U.S. legally.

José Antonio Cruz had lived in the U.S. illegally for 18 years before he was deported after being detained for speeding in 2018. He left his hometown in central Mexico last week in hopes of crossing the border in coming weeks and restarting his small construction business in Texas.

"The economy is going to reactivate very quickly in the U.S. They are already reopening, so it is a very good moment to be there," Mr. Cruz, 41, said by telephone from a Catholic shelter in northern Mexico.

It's unclear how many people are making it into the U.S. by crossing the border illegally. Estimates compared with how many are apprehended vary widely. A 2017 Homeland Security report estimated that between 55% to 85% of immigrants crossing the border illegally in 2016 were apprehended.

During the past seven days, border officials estimated that about 6,500 people evaded detection while entering the U.S., according to a person familiar with the government's internal estimates. A week ago, border agents had recently averaged about 5,000 arrests a day, according to internal Homeland Security documents reviewed by The Wall Street Journal.

Migrants and immigration analysts offer several explanations for this year's surge. The U.S. is hiring after a long and brutal pandemic, while Mexico lost some 2.4 million jobs last year.

Additionally, some migrants say there is a widespread perception that it's become easier -- and more inviting -- to enter the U.S. under President Biden than under former President Donald Trump.

"Expectations were created that with the government of President Biden there would be a better treatment of migrants," Mexican President Andrés Manuel López Obrador said at his daily press conference on Tuesday. "And this has caused Central American migrants, and also from our country, wanting to cross the border thinking that it is easier to do so."

The Department of Homeland Security said in a statement, "There is no change in policy: the border remains closed. Families and single adults are being expelled under Title 42 and should not attempt to cross illegally." The department added that some families will be released into the U.S. if Mexico doesn't accept their return and there is no bed space available at Immigration and Customs Enforcement facilities.

Title 42 refers to a public-health law used by the Trump administration at the beginning of the pandemic to turn nearly all single adults back at the border rather than detaining or formally deporting them. That makes it easier to try to cross again -- leading to another factor in the increase.

The Biden administration has curtailed many of the Trump administration's enforcement policies focused on those already in the U.S. Immigration and Customs Enforcement officers and agents have been told to focus their resources on recent border crossers, serious criminals and those who pose a threat to national security or public safety.

The pandemic and stay-at-home policies throughout the region have created pent-up demand to migrate, said Theresa Cardinal Brown, director of immigration and cross-border policy at the Bipartisan Policy Center, a Washington think tank. She said the factors that cause people to migrate, such as poverty, have only gotten worse during the pandemic.

"The policies at the border have come and gone and changed, but the migrants keep coming," she said.

Adrián Cahun, who came to the U.S. illegally and worked in San Francisco restaurants as a dishwasher and bartender for six years before returning to his rural hometown of Oxkutzcab in 2008, said he plans to again cross the border as soon as the pandemic ends.

"If there's a recovery in the U.S., it's easier to go there and find work," said Mr. Cahun. He had saved enough money while he was in the U.S. to build a house and open a cafeteria in Oxkutzcab. With few affluent customers there, he said, he closed the cafeteria a few years ago. He currently works filling shelves at a supermarket.

"Many were afraid of Trump and most didn't want to try to cross the border," he added. "But now many here are thinking of emigrating to the U.S."

Last March, as the pandemic took hold, the Trump administration invoked a 1944 public-health law, Title 42, to immediately expel any migrant caught crossing the border back to Mexico without processing them at cramped detention facilities, where Covid could spread.

The change, particularly for single adult males, marked a departure from border policies under former presidents since George W. Bush.

Before the 2000s, most Mexicans caught at the border had been allowed to return home with little consequence, and other migrants were typically allowed into the U.S. with orders to report back to immigration authorities later.

Then the U.S. started formally deporting most migrants caught at the border. In some Border Patrol sectors, adult crossers were charged with a criminal offense and often spent weeks or months in jail before being deported. Any further attempt to cross could result in more jail time in the U.S.

During the Obama administration, migrants from Mexico were often sent to border cities far from where they had been caught, or flown to the interior of Mexico. Migrants from countries other than Mexico were often flown back home.

Under Title 42, nearly all single adult migrants remain at the border after being turned away. Many try to re-enter. The percentage of migrants caught at the border who had already been caught once grew to nearly 40% during the past six months, compared with 7% in 2019.

"That's the wonderful thing now. You have the opportunity to bat again and again. That's better for us," said Lucio Portillo, a 44-year-old Honduran migrant who is now in Mexico planning to cross to the U.S. in search of work.

Sara Abdala, who manages a shelter in Altar, a migrant hub some 70 miles from the Arizona border, said she knew a Honduran migrant who recently tried six times in two weeks until he managed to enter the U.S. without being detected.

"The town has come back to life in recent months after it was almost empty during the pandemic. The business of migration has become hot again," she said.

Victor Manjarrez, a former chief patrol agent in both El Paso and Tucson, said reports from the border remind him of his days patrolling southern Arizona in the early 2000s, when that sector was the border's busiest. The majority of illegal border crossers then were single adults from Mexico.

"There isn't that idea of an impact, of a consequence," said Mr. Manjarrez, now an associate director at the University of Texas and El Paso's Center for Law & Human Behavior. "That's what I would be telling my troops -- that consequences work."

The Biden administration hasn't announced plans for the fate of Title 42. The American Civil Liberties Union has sued the government, arguing the practice illegally blocks migrants from asking for asylum or other protections in the U.S. Both sides are trying to negotiate a settlement.

The Biden administration had initially followed the Trump administration approach of returning families caught at the border back to Mexico, too. But that changed after Mexico's government, at the request of Unicef, barred the practice of holding the children and families at Mexican federal detention centers, handing them over instead to a family welfare agency that has far less capacity.

Though some families are still being returned to Mexico, most are being released into the U.S., where federal law prohibits families with children from being held more than about 20 days. Space in family detention centers has been limited because of the pandemic, though Immigration and Customs Enforcement announced this week a new $86.9 million contract for about 1,200 beds.

Acting ICE Director Tae D. Johnson said in a statement that the contract includes money for Covid-19 testing and other health screenings.

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03-24-21 1836ET