SAN DIEGO — With border crossings at a 40-year low, the U.S. Border Patrol announced a new strategy Tuesday that targets repeat crossers and tries to find out why they keeping coming.

For nearly two decades, the Border Patrol has relied on a strategy that blanketed heavily trafficked corridors for illegal immigrants with agents, pushing migrants to more remote areas where they would presumably be easier to capture and discouraged from trying again.

The new approach is more nuanced. Outlined in a 32-page document that took more than two years to develop, agents will now draw on intelligence to identify repeat crossers and others perceived as security threats, Chief Mike Fisher said.

“This whole risk-based approach is trying to figure out who are these people? What risk do they pose from a national security standpoint? The more we know, the better informed we are about identifying the threat and potential risk,” he said.

During testimony before a House Homeland Security subcommittee Tuesday, Fisher was repeatedly asked why the new strategy didn’t include any specific “metrics” that could help members of Congress and the public better understand if the border is secure.

“How can you possibly measure if the border is secure at all?” asked Texas Republican Michael McCaul.

Fisher defended the strategy, saying it will help agents use the resources at the border to better understand what is really happening.

Conditions on the border have changed dramatically since the last national strategy, putting pressure on the agency to adapt to a new landscape.

An unprecedented hiring boom more than doubled the number of agents to 21,000 since 2004, accompanied by heavy spending on fencing, cameras and sensors.

At the same time, migration from Mexico has slowed significantly. Last year the Border Patrol made 327,577 apprehensions on the Mexican border, down 80 percent from more than 1.6 million in 2000. It was the slowest year since 1971.