A U.S. Border Patrol agent monitors a group of men shortly after they were apprehended while crossing into the United States illegally. (Loren Elliott/AFP/Getty Images)

The Department of Homeland Security said a surge of illegal crossings last month justifies the Trump administration’s call for an emergency deployment of National Guard troops to the Mexican border, citing a 37 percent increase in the number of people taken into custody by the U.S. Border Patrol.

Federal agents arrested or denied entry to 50,308 unauthorized migrants in March, the highest one-month total since President Trump took office and a 200 percent increase over the same period last year, when crossings fell to historic lows.

The monthly arrest figures are widely used as a gauge of illegal migration patterns, and the numbers typically rise each spring as demand for labor increase at farms and other worksites. But this year’s jump between February and March was the largest of the past five years, and driven almost entirely by migrants from Honduras, Guatemala and El Salvador.

In a statement, DHS spokesman Tyler Q. Houlton said Thursday that the sudden increase was evidence “the crisis at our Southwest border is real.”

“Illegal aliens continue to exploit our immigration laws,” Houlton said. “We need to close these dangerous loopholes that are being taken advantage of each and every day, gain operational control of the border, and fully fund the border wall system,” he added, referring to the $25 billion Trump border security plan that has stalled in Congress.

Houlton’s mention of loopholes was a reference to the Trump administration effort to tighten asylum procedures it says migrants are exploiting. The backlog of asylum claims in the U.S. immigration court system is so lengthy that applicants who enter the country without authorization can spend several years living and working in the United States while their claims are adjudicated.

The DHS said it registered a 49 percent increase in the number of families taken into custody last month, a category consisting of a child and at least one adult relative. The number of unaccompanied minors under age 18 rose 41 percent. The totals for those categories were still below their 2014 peak, but administration officials said they pointed to the urgency of taking swift action.

Administration officials say they are considering measures to discourage illegal migration as the weather warms, and since last year the DHS has threatened to separate parents from their children to deter them from attempting the trip.

The agency has implemented the practice on a limited basis in recent months, drawing criticism from immigrant activists and child welfare advocates.

The DHS typically releases the monthly border apprehension totals later in the month, and their publication a few days early indicated the Trump administration was seeking to bolster its claims that a National Guard deployment of up to 4,000 troops is warranted. 

“I usually start looking for the numbers to be released around the ninth or 10th of each month,” said Adam Isacson, who tracks border security at the Washington Office on Latin America. “These were out on the fifth.”

The threats of family separation and a tightening of asylum rules could be one explanation for the surge, Isacson said. “Rumors about a coming crackdown could be making people try to take the trip now, especially if that’s the message of some of the smugglers,” he said.

Arrests along the border with Mexico have been falling for the past decade and reached their lowest level since 1971 last year.

“Because of the Trump Administrations actions, Border crossings are at a still UNACCEPTABLE 46 year low,” the president wrote on Twitter Thursday.