The figures show a sharp drop in apprehensions immediately after President Trump's election win, possibly reflecting the deterrent effect of his rhetoric on would-be border crossers; starting in May, the number of people taken into custody began increasing again.
Arrests of foreigners living illegally in the United States have surged under Trump. Immigration and Customs Enforcement officers made 110,568 such arrests between inauguration and the end of September, according to the figures published Tuesday, a 42 percent increase over the same period during the previous year.
Tom Homan, ICE's temporary director and Trump's nominee to lead the agency, praised the president and gave a vigorous defense of ICE's more aggressive approach.
"This president, like him or love him, is doing the right thing," Homan told reporters at a news conference in Washington, accompanied by the heads of the U.S. Border Patrol and Citizenship and Immigration Services.
"A 45-year low in border crossings? That's not a coincidence," Homan said. "That's based on this president and his belief and letting the men and women of ICE and the Border Patrol do their job."
Trump's sweeping promises to crack down on illegal immigration fueled his presidential campaign and are at the center of his most ambitious domestic policy proposals, including construction of a wall along the border with Mexico.
Asked whether such a barrier was justifiable given its high cost and the decline in illegal immigration, DHS officials endorsed the president's plan.
"In this society, we use walls and fences to protect things. It shouldn't be different on the border," said Ronald Vitiello, chief of the Border Patrol.
Apprehensions by Border Patrol agents peaked at more than 1.6 million in 2000 and began falling substantially after 2008. The previous low point was 331,333 arrests, during fiscal 2015. Experts have attributed the decline to tougher U.S. enforcement, improving job prospects in Mexico and long-term demographic changes that have driven down the country's birthrate.
Still, the drop in border arrests is among the sharpest year-to-year changes on record, one that only casts more doubt on the wisdom of building a border wall, said Doris Meissner, senior fellow at the Migration Policy Institute, a Washington think tank.
"It's a throwback response to yesterday's problems," she said, arguing that the money would be better spent addressing what accounts for a growing share of illegal migration: families with children fleeing rampant violence and dismal poverty in Central America.
Border agents took more than 75,000 "family units," classified as at least one child and a related adult, into custody during fiscal 2017. But the number of unaccompanied minors fell 31 percent, to 41,435.
Despite making fewer arrests and picking up more families, Vitiello said, border officers face growing dangers on the job. During fiscal 2017, Border Patrol agents were assaulted 847 times, a 45 percent jump, according to the agency's statistics. Vitiello said the change could be the result of increasing resistance to arrest by migrants frustrated by the difficulty they face now in reaching the United States.
He said the use of firearms by agents dropped to a record low of 17 incidents during the period, down from 55 in 2012, when a spate of fatal cross-border shootings forced the agency to change its use-of-force policies. Vitiello said those numbers were evidence the changes were working.
The statistics released Tuesday say Mexican nationals account for a diminishing share of those taken into custody along the border. During fiscal 2017, 58 percent were from countries "other than Mexico," CBP reported, led by El Salvador, Guatemala and Honduras.
Of those arrested, 10 percent had been apprehended on at least one other occasion, down from 12 percent the year before. DHS officials use this "recidivism" rate as a gauge of how difficult it is to cross into the United States illegally, and recent DHS studies have concluded that the border is tougher than ever to sneak across.
Trump has asked Congress to fund 10,000 more ICE officers and an additional 5,000 Border Patrol agents to meet expanded enforcement goals.
Tuesday's data release also provides the clearest look at how the Trump administration is tightening immigration far beyond the border.
Soon after the election, Trump pledged to deport or incarcerate "probably 2 million" foreigners with criminal records who he said were gang members and drug dealers, saying the number could be "as high as 3 million."
But the number of people ICE removed from the United States declined about 6 percent during the government's 2017 fiscal year, in part because the drop in illegal border crossings left the agency with fewer people to deport.
ICE said the number of "criminal aliens" it deported increased to 67,859 during fiscal 2017, a 12.5 percent increase over the previous year.
But the number of arrests made by ICE agents for "administrative" reasons surged 30 percent. Such arrests target a category of foreigners whose offenses are typically immigration-related violations as opposed to conventional criminal charges.
Homan said ICE is simply enforcing the country's existing immigration laws with a rigor that was lacking under the Obama administration.
"There's no population that's off the table," he said. "If you're in the country illegally, we're looking for you and we're looking to apprehend you."
Attorneys and rights advocates say the administration has cast a cloak of fear over immigrant communities, especially the parents of U.S.-born children who worry they'll be separated. "There is a terrible effect on children when their parents face deportation," said Lauren Dasse, executive director of Arizona's Florence Immigrant and Refugee Rights Project, which provides legal assistance to immigrants in ICE custody.
Dasse said her group has been holding more workshops for parents worried they will lose custody of their children if they're picked up by immigration agents.
Customs and Border Protection also published its end-of-year drug-seizure totals Tuesday. The numbers show a stunning increase in seizures of the deadly synthetic opioid fentanyl in recent years, from 2 pounds in 2013 to 1,485 pounds this past fiscal year.
The amount of cocaine, heroin and methamphetamine confiscated along the border rose, as well, while marijuana declined.