A recent decline in arrests for illegal crossings on the U.S. border with Mexico may prove only temporary. The drop in January reflects how the numbers ebb and flow, and the reason usually goes beyond any single factor.

After a record-breaking number of encounters at the southern border in December, crossings dropped by half in January, authorities reported Tuesday. The largest decrease seen was in the Del Rio sector encompassing Eagle Pass, Texas, the main focus of Republican Gov. Greg Abbott’s recent border enforcement efforts. Mexico also increased enforcement efforts during that time after talks with U.S. President Joe Biden’s administration.

A look at the numbers and what’s behind them:

WHAT DO THE NUMBERS SAY?

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Overall, arrests by Border Patrol dropped in January by 50% from 249,735 in December, the highest monthly tally on record.

Tucson, Arizona, was again the busiest sector for illegal crossings with 50,565 arrests, down 37% from December, followed by San Diego. Arrests in the Border Patrol’s Del Rio sector, which includes the city of Eagle Pass, plummeted 76% from December to 16,712, the lowest since December 2021. Arrests in Texas’ Rio Grande Valley, dropped 60% to 7,340, the lowest since July 2020.

A significant decrease was noted among Venezuelans whose arrests dropped by 91% to 4,422 from 46,920. But those numbers could change soon. Panama reported that 36,001 migrants traversed the dangerous Darien Gap in January, up 46% from December. The vast majority who cross the Panamanian jungle are Venezuelans headed to the United States, with considerable numbers from Haiti, China, Ecuador and Colombia.

WHAT IS MEXICO DOING?

Mexico has been forcing migrants from freight trains that they sometimes use to cross the country to get closer to the U.S. border. Immigration officers in Mexico also has been busing migrants to that country’s southern border and flying some back to their countries.

That enforcement effort began after a visit from U.S. Secretary of State Antony Blinken to Mexico City on Dec. 28.

Mexican border states such as Coahuila partnered with Mexico’s federal government. By January, members of Mexico’s military and national guard were patrolling the banks of the Rio Grande. Officers filled buses with migrants and drove them away from Piedras Negras, which is on the Mexican side of the river across from Eagle Pass.

WHAT IS TEXAS DOING?

Over in Eagle Pass, the Texas National Guard took over a city-owned park along the river. Texas has denied U.S. agents access to Shelby Park since Jan. 10. It also installed additional razor wire and anti-climbing fencing in the area.

Border Patrol agents had previously used the park for monitoring and patrols, as well as to process migrants who made it across the river to U.S. soil. Migrants who are seeking asylum are released to await immigration court proceedings that can take years.

“What you have is this magnet,” Mike Banks, Texas’ border czar, said.” You’re basically saying, `Cross the river right here. Get across and we’ll process immediately and release you.’ So again, that’s a pull factor. So we’ve taken that pull factor away.”

WHAT ELSE IMPACTS THE NUMBERS?

The number of people trying to make the journey often increases when the weather is warmer in the U.S. and decreases during the colder months, including January.

Since 2021, crossings on the southern border increase by an average of 40% from January to March, according to federal data from the last three years.

Another factor last year was the end of COVID-19 restrictions in May. The use of a public health policy known as Title 42 allowed the Trump and Biden administrations to turn asylum-seekers back to Mexico, even if they were not from that country.

Crossings fell dramatically for a month after Title 42 ended and the Biden administration enforced new rules.

Under Title 42 migrants were denied asylum more than 2.8 million times starting in March 2020 on grounds of preventing the spread of COVID-19. When it expired, the Biden administration launched a policy to deny asylum to people who travel through another country, like Mexico, to the U.S., with few exceptions.

However, the numbers eventually started climbing until reaching a record high in December.