SAN DIEGO (AP) — Dozens of Marines unfurled coils of concertina wire — one on the ground and one slightly above — making it more difficult to climb a border wall separating Tijuana from San Diego. They worked with speed and efficiency amid a weekend rush of cars nearby at the busiest border crossing between the U.S. and Mexico.

Fortifying barriers has long been a military task on border missions that date back to the administration of George W. Bush. But President Donald Trump has hinted strongly at the unprecedented use of the armed forces to repel what he calls a “disastrous invasion.”

Until now, the military has limited itself to a supporting role at the border — surveilling for illegal crossings by ground and air, repairing vehicles, building barriers — adhering to the Posse Comitatus Act of 1878 to keep the armed forces away from civilian law enforcement.

Trump’s declaration of a national emergency at the southern border indicates he may redirect money, as he did during his first term, to get billions of dollars for border wall construction.

His inaugural day orders raise the possibility of invoking wartime powers, including the Insurrection Act of 1807, allowing him to deploy active-duty troops to suppress a rebellion. He gave Defense Secretary Pete Hegseth and Homeland Security Secretary Kristi Noem 90 days to deliver recommendations.

The Insurrection Act “is just all-purpose. The regular military can do anything on U.S. soil,” said Adam Isacson, who follows the role of military at the Washington Office on Latin America, a research and advocacy organization.

Trump has already broken from predecessors. While the military has housed migrants at times, its deportation flights to Guatemala, Ecuador and Colombia mark a departure from previous administrations. Trump said he would use a detention center at Guantanamo Bay, Cuba, to hold tens of thousands of the “worst criminal aliens,” though it hasn’t happened yet.

There is a sense among Border Patrol agents and others that there is more to come. Isacson said he believes the administration may see a model in Texas Gov. Greg Abbott’s “Operation Lone Star,” which thrust the state’s National Guard into a central role in border enforcement.

The Defense Department deployed 1,600 active-duty troops to the border during Trump’s first week in office. U.S. officials said Friday that they are preparing to deploy at least 1,000 more in Trump’s immigration crackdown, about half to the border and half to Guantanamo Bay.

So far, the military has maintained low visibility in San Diego. The Marines laid concertina wire Friday at the bottom of an 18-foot-high (5.5-meter-high) border wall that already had wire on top. Migrants who manage to get over or through the wire face a second, 30-foot-high (9.1-meter-high) wall.

A tour in Osprey military transport aircraft, which have been used to bring concertina to Brown Field Municipal Airport in San Diego, showed Border Patrol vehicles staged at various lookouts. They stretched about 70 miles (112 kilometers) from the Pacific Ocean through boulder-strewn ranches east of San Diego and a treacherous mountain range where few migrants cross.

Border Patrol Chief Michael Banks said Thursday that arrests for illegal crossings averaged 654 a day over the previous week, down from a daily average of 1,527 in December.

San Diego has been the busiest corridor for illegal crossings over much of the last year. Arrests averaged 222 a day in a seven-day period through Jan. 25, down from 237 the previous week.