SAN DIEGO (AP) — House Minority Leader Kevin McCarthy said Monday during a visit to El Paso, Texas, that, “It's more than a crisis. This is human heartbreak.” Homeland Security Secretary Alejandro Mayorkas on Tuesday called the wave of migrants a difficult challenge but nothing new.

Spin and semantics aside, migration flows to the U.S. from Mexico are surging in a major way for the third time in seven years under Republican and Democratic presidents — and for similar reasons.

HOW HAVE FLOWS CHANGED SINCE JOE BIDEN BECAME PRESIDENT?

Border encounters — a widely-used but imperfect gauge that tells how many times U.S. authorities came across migrants — rose sharply during Donald Trump's final months as president, from an unusually low 17,106 last April to 74,108 in December. Last month, encounters topped 100,000 for the first time since a four-month streak in 2019.

That's only part of the picture, though. Who's crossing is just as important a gauge as how many are making the attempt, if not even more.

Mexican adults fueled last year's rise, a throwback to one of the largest immigration waves in U.S. history, from 1965 through the Great Recession of 2008. Last March, the Trump administration introduced pandemic-related powers to immediately expel people from the United States without an opportunity to seek asylum. Facing no consequences, Mexican men kept trying until they made it.

The percentage of encounters that were repeat crossers hit 38% in January, compared to a 7% rate in the 12-month period that ended in September 2019. The recidivism rate was 48% among Mexican adults during one two-week stretch last year in San Diego.

Families and children traveling alone, who enjoy more legal protections and require greater care, became a bigger part of the mix after Biden took office. They accounted for 29% of all encounters in February, up from 13% two months earlier.

The Border Patrol encountered 561 unaccompanied children on Monday, up from an average daily peak of 370 during Trump's presidency in May 2019 and 354 during a peak in Barack Obama's presidency in June 2014. A U.S. official provided Monday's total to The Associated Press on condition of anonymity because it was not intended for public release. The daily average was 332 in February, up 60% from a month earlier.

WHY ARE FAMILIES AND CHILDREN SUDDENLY COMING NOW?

Trump, responding to a massive increase in Central American families and children that peaked in May 2019, expanded his “Migrant Protection Protocols” policy to force asylum-seekers to wait in Mexico for hearings in U.S. immigration court. It was unquestionably effective at deterring asylum — less than 1% have won their cases, according to Syracuse University's Transactional Records Access Clearinghouse — but asylum-seekers were exposed to violence in Mexico, as documented by advocacy group Human Rights First and others. Attorneys were extremely difficult to find in Mexico.

Other Trump-era policies included fast-track asylum proceedings inside U.S. Customs and Border Protection holding facilities, where access to attorneys was next to impossible. Agreements were struck with Guatemala, Honduras and El Salvador for the U.S. to send asylum-seekers to the Central American countries with an opportunity to seek protection there instead.

Biden quickly jettisoned those Trump policies as cruel and inhumane, making good on campaign promises. He has kept in place Trump's pandemic-related expulsion powers but exempted children traveling alone.

Biden wants Congress to give $4 billion to address root causes of migration in Central America such as poverty and violence, which have driven people to the U.S. for decades, including a surge of children in 2014.

WHAT IS THE BIDEN ADMINISTRATION DOING?

In addition to ending Trump policies and seeking foreign aid, the Biden administration wants to speed the release of children to parents, relatives and others in the United States, avoiding detention conditions that drew widespread criticism during surges in 2014 and 2019.

The administration was scheduled to begin processing unaccompanied children as early as Wednesday at the Dallas Convention Center, days after establishing a makeshift facility in Midland, Texas. The U.S. official who spoke to the AP said the U.S. Department of Health and Human Services was looking at additional holding facilities at Moffett Federal Airfield, near San Francisco, and in Pecos, Texas, as well as expanding into Donna, Texas, in a joint effort with Customs and Border Protection.

Nearly 1,900 of about 2,500 unaccompanied children in custody in the Rio Grande Valley on Monday were there longer than the 72-hour limit established in agency policy, the official said.

About seven of every 10 encounters in February resulted in expulsion under pandemic powers, limiting need for detention space. Mexican and Central American adults and families were sent back to Mexico. Mexican authorities have resisted taking back Central American families from Texas' Rio Grande Valley, the busiest corridor for illegal crossings, prompting U.S. authorities to fly them to El Paso, Texas, and San Diego to be expelled.

Others picked up at the border may be released in the United States with notices to appear in immigration court.

The Biden administration is also stepping up efforts to have children apply for asylum from their homes in Central America instead of making the dangerous journey to the U.S. border.