The U.S. government now patrols nearly half the Mexican border by drones alone in a largely unheralded shift to control desolate stretches where there are no agents, camera towers, ground sensors or fences, and it plans to expand the strategy to the Canadian border.

It represents a significant departure from a decades-old approach that emphasizes boots on the ground and fences. Since 2000, the number of Border Patrol agents on the 1,954-mile border more than doubled to surpass 18,000 and fencing multiplied nine times to 700 miles.

Predator Bs sweep remote mountains, canyons and rivers with a high-resolution video camera and return within three days for another look. That's according to two officials with direct knowledge of the effort on condition of anonymity because details have not been made public.