The area is part of the nation's busiest Border Patrol sector, which received additional agents and fencing as the federal government sought to improve border security in recent years.
To sneak drugs into the country, teams of smugglers who lug up to 40 pounds of marijuana each in backpacks made of burlap and flour bags typically have to either walk over the border or scale the fence that covers about 30 percent of the 2,000-mile U.S.-Mexico border.
Agents patrol the fence by driving along roads beside the barriers and monitor the area through surveillance cameras mounted on nearby towers and sensors placed along smuggling routes after the fence.
Agents who may find it hard to spot smugglers in the brush use remote sensors to detect illegal border-crossers so agents can be deployed in response. Seismic sensors buried in the ground on paths and trails in the desert and mountains north of the border are used to detect the passage of people, animals and vehicles.
Each Board Patrol station monitors hundreds of such sensors and can dispatch agents to the scene when the devices are triggered. "As people walk over that, that will trigger the signal," McCubbin said. "You go out and check it out."
Multiple sensors can be deployed along a path so agents can tell the direction of travel and get an idea of how large a group is involved, he said. Then agents can either get ahead of the group or come in from behind and use night-vision goggle sand infrared cameras to spot them.
Border Patrol agents often are posted several hundred feet and even miles away from the border to look for people who sneaked into the country. Once smugglers get past several layers of enforcement, they typically hook up with a driver on a highway or ranch road and use back roads to make their way around Border Patrol checkpoints.
In the county where the shooting occurred, checkpoints could be seen on two highways that carry traffic to and from Interstate 10.
Drug-sniffing dogs and agents screen traffic passing through the checkpoints, resulting in searches of vehicles if there are indications of smuggling activity. But the checkpoints' real enforcement purpose is indirect, Stoddard said.
"The real purpose of the checkpoint is to get the dope or people on foot in a remote area where they can be picked up. It's gravy when a load goes into the (checkpoint)."