Between October and April, Mexican authorities detained 92,889 Central Americans, up from 49,893 over the same period the previous year.
Mexico's arrest totals far exceeded the 70,226 migrants caught by U.S. border agents over the past six months who were classified as "other than Mexican," the majority of whom are typically from Guatemala, El Salvador and Honduras.
And it marks a huge shift in comparison to the same period during the previous year, when U.S. agents were overwhelmed by a sudden influx of Central American children, and the United States apprehended 159,103 non-Mexican border crossers.
So far this year, the number of detained migrants registered as "unaccompanied children" is down by 51 percent, about the same as the number of "family units" caught crossing illegally.
A big reason for the decline is Mexico's "Southern Border Plan," which has assigned 5,000 federal police agents to intercept northbound migrants at highway checkpoints and along railway routes. The program is a response to both pressure from the United States and human rights groups, as well as a Mexican public appalled by years of unchecked abuses against Central American migrants.
By stopping Central American travelers from reaching the U.S. border and attempting to cross — especially in the deadly heat of summer — officials in both countries say they are protecting them from criminal gangs and a perilous hike through the desert.
Tougher enforcement in Mexico spares the United States the cost of detaining, processing and repatriating migrants on charter flights to Central America, an expense that has increased with the growth in petitions for asylum from those seeking protection from endemic violence back home.
But Mexico's newfound enforcement enthusiasm is raising fears that detained migrants are being rushed back to Central America with little concern for the threats they may face.
"Not all migrants qualify for international protection, but the Mexican government needs to ensure that it is screening all migrants so that those that do qualify are not merely being returned to the violence they were fleeing from," said Maureen Meyer, a Mexico researcher at the Washington Office on Latin America (WOLA), a D.C. think tank.
It isn't the first time Mexico has attempted such a crackdown, but in the past, tougher enforcement was often viewed as a kind of favor to the United States, especially because the Central Americans rarely remained in Mexico.
At the same time, Mexican illegal immigration to the United States has plunged, and the number of Mexicans leaving the United States as deportees or for other reasons is now believed to be roughly equal to the number of new migrants who arrive.
Today, though, human smuggling and "toll collecting" by extortionists along the border have become a lucrative side businesses for the criminal mafias battling the Mexican government, giving the country's leaders an additional incentive to choke off the profits.
Central to Mexico's enforcement strategy is its increased police presence along the freight rail routes that for years have carried migrants north, but became rolling terror wagons, where the poorest and most vulnerable travelers were routinely raped and kidnapped by gangs or mutilated in grisly accidents.
Mexico's Southern Border Plan "is hunting down migrants and not allowing them to travel on the train," said Marta Sanchez of the Mesoamerican Migrant Movement, an advocacy group.
With many Central Americans still determined to get to the United States — and in many cases, reunite with their families up north — advocates say the crackdown is pushing travelers away from church-run shelters and advocacy organizations that otherwise protect them.
"A humanitarian support system was set up to provide services to the migrant population riding on the train and to register complaints," said WOLA's Meyer. "With migrants taking new, and often riskier routes, including on boats on the Pacific Coast, there are less services available to them and they are even more invisible and vulnerable than before."
Nor has the crackdown ended the violence against migrants. Earlier this month, Mexican police found three bodies in burned vehicles at a ranch just south of the Arizona border where more than a dozen migrants were being held at gunpoint.
The Obama administration has asked Congress for a $1 billion aid package for Central American security and economic development, funds he says are critical to give the region's would-be migrants more incentive to remain in their home countries.