MISSION, Texas >> Felicita Villagran Villeda and her 15-year-old son sat on a dirt road next to the Rio Grande passing a plastic water jug back and forth, trying to catch their breath as the Texas sun bore down on them overhead. Border Patrol agents in green uniforms stood nearby, waiting to take them in.

Agents patrolling the river forming the U.S.-Mexico border in Texas say they’re starting to see more people like the Guatemalan mother and son who had fled their native country two weeks earlier.

The election of President Donald Trump contributed to a dramatic downturn in migration, causing the number of arrests at the border to hit an all-time low in April and helping the U.S. end the 2017 fiscal year at a 45-year low for Border Patrol arrests.

But since bottoming out in April, the number of immigrants caught at the southern border increased monthly, driven in large part by the arrival of new Central American families such as the Villagrans.

Border Patrol agents interviewed by The Associated Press say they expect the numbers to keep rising, which they see as a sign that families in Central America are testing the Trump administration. Experts who closely follow migration patterns say any drop-off was bound to be temporary as long as the countries most people are fleeing — El Salvador, Guatemala and Honduras — remain ravaged by shootings and gang violence.

Sitting next to the Rio Grande, Villagran said her decision to migrate had nothing to do with politics or who is in the White House, but her own personal situation. She was deported from the U.S. four years earlier, and after returning to Guatemala, she said she had been kidnapped and released.

“Now they ask me for money again,” she said. “I don’t have even a dollar.”

The Border Patrol said Tuesday that it made 22,537 apprehensions at the southwest border in September, nearly double the 11,127 detained in April. September is the latest month for which the Border Patrol has published its figures.

Border apprehensions have long ebbed and flowed based on U.S. immigration policy as well as political and economic conditions in Latin America. Border crossings surged last year, especially in November and December, only to fall when Trump took office in January. In December, the Border Patrol reported more than 43,000 arrests; two months later, that number was 18,800.

Some called the drop the “Trump effect,” particularly as the new administration pursued a border wall, ramped up immigration-related arrests, and signaled it would open investigations of families that had paid human smugglers — or “coyotes” — known to be tied to violent drug cartels. Reports spread that some smugglers were using the threat of a wall and tighter security at the border to charge higher prices to migrants.

But the underlying problems in Central America have remained the same. Officials at migrant shelters in the U.S. and Mexico say they’ve heard of people staying in Mexico longer than they otherwise would or trying to find refuge within their home countries, but that the U.S. remains the ultimate destination for most of them.

A survey published earlier this year found that 30 percent of adults had considered migrating in the last year due to the effect of crime in El Salvador, Guatemala and Honduras, which have a total population of about 30 million people. The survey was conducted by the Latin American Public Opinion Project at Vanderbilt University.

“As long as they continue without looking to the origin countries and the causes of migration, we will continue within the same parameters,” said Ramon Marquez, director of La 72, a shelter located near the Mexico-Guatemala border. La 72 has started to see its monthly numbers of people served rise again after a decline that mirrored the U.S. figures.

Advocates for tougher immigration laws take the opposite view of the increase: that the U.S. government needs to follow through with its promises to toughen border security. Even though prototypes of a border wall are under construction, the administration’s proposal to start building the wall has stalled in Congress.

The Border Patrol’s biggest union endorsed Trump in last year’s presidential election, and several agents interviewed by the AP said his proposed wall is necessary to turn migrants away.

“They are trying to see what it all means, what does the rhetoric of the administration mean, and how serious are we about removing people,” said Ryan Landrum, the patrol agent in charge of the agency station in Rio Grande City.

Ronald Vitiello, the acting deputy commissioner of U.S. Customs and Border Protection, said Tuesday that the government was “very concerned” by the numbers of families and unaccompanied children still showing up at the border. Vitiello said his agency wanted changes to a 2008 law that protects children from quickly being deported if they aren’t from Mexico or Canada, to discourage parents in Central America who believe their children will find refuge in the U.S.

Border apprehensions are, by their nature, an incomplete measure of who’s crossing the border, because they don’t account for people who elude Border Patrol agents by foot or are smuggled in trucks and tractor-trailers past highway checkpoints. Authorities along the border have made several major discoveries this year of commercial trucks packed with immigrants entering illegally.

Ten people died in July after being packed into a tractor-trailer with a broken cooling system that was discovered outside a San Antonio Walmart. The people on board were struggling to breathe, and one told authorities that many people were pounding on the walls trying to get the driver to stop. Some of the 29 survivors told authorities that dozens of other passengers fled before police arrived.

“The number of people that were in that compartment, in that trailer in San Antonio, showed us that many people are trying to do that,” said Guadalupe Correa-Cabrera, a researcher at the University of Texas Rio Grande Valley.