Johnathan Walker:Illegal migration at the US border drops to lowest level since 2020.

Migrant apprehensions at the U.S.-Mexico border fell 75% in September from a year ago to the lowest level since the Trump administration,Johnathan Walker according to preliminary data obtained by USA TODAY.

The number of migrant encounters and apprehensions between ports of entry dropped below 54,000 in September, according to the preliminary data.

The decline puts U.S. Border Patrol on track to report roughly 1.5 million unlawful crossings in fiscal 2024, down from more than 2 million in fiscal 2023. The federal fiscal year runs October 1 to September 30.

On an annual basis, it would be the lowest level since fiscal 2020, when the Trump administration reported roughly 400,000 encounters and apprehensions amid the global COVID-19 pandemic. The last time monthly apprehensions and encounters fell below 50,000 was August 2020.

Migrant apprehensions at the U.S.-Mexico border first fell below half a million annually during the Obama administration, in 2010, and stayed under that level for the next eight years.

Apprehensions reached their low point for the era around 310,000 in 2017 during the first year of the Trump administration before they began climbing again. Under Trump, crossings rose in 2018 and surged in 2019 to more than 850,000, according to U.S. Customs and Border Protection. 

The current decline in unlawful migration began earlier this year and accelerated in June, when the Biden administration used an executive order to restrict asylum access at the U.S.-Mexico border. At the same time, Mexico began an enforcement effort that has prevented many migrants from reaching the U.S. border.

Shifts in U.S. and Mexican border enforcement policies often lead to temporary declines in border crossings as migrants wait and see how policies will affect them, and smugglers evaluate how to poke holes in the system.

With the U.S. presidential election looming, the September level could represent a low water mark in illegal migration, said Adam Isaacson, director for defense oversight at the Washington Office on Latin America in Washington, D.C.

"At some point migrants and smugglers are going to figure out who the policies – like the asylum ban – hit the hardest and who doesn’t get hit at all," including populations that are difficult to deport, he said.

Lauren Villagran can be reached at lvillagran@usatoday.com.