That's up from 331,333 people the previous year, but down from 2014 and 2013.
Most of those apprehended in the last year came from Honduras, El Salvador and Guatemala, countries struggling with drug-related violence and extreme poverty.
Net migration from Mexico has remained in negative territory for several years, according to U.S. and Mexico census data, with more people returning home to Mexico than entering the U.S. illegally each year.
The focus on the wall that Trump has promised to build comes amid growing confusion about who would pay for it. Trump has suggested it would cost about $10 billion, but outside estimates are double or triple that.
After repeatedly vowing on the campaign trail that he would force Mexico to shoulder the cost, Trump in October suggested he would get Mexico to reimburse the costs -- a step Mexican officials have said is out of the question.
After Trump's aides signaled last week that he would ask Congress to find the money, Trump tweeted, "The dishonest media does not report that any money spent on building the Great Wall (for sake of speed), will be paid back by Mexico later!"
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