He explains that the spotters sit on peaks all the way from the border to Phoenix. They outnumber the Shadow Wolves and are equipped with night vision goggles, mobile phones and radios that deliver encrypted messages to drug mules on the ground. Other spotters work for people smuggling gangs and are in touch with the "coyotes" who guide groups of illegal immigrants across the desert.
"We're probably being watched right now," says Mr Garcia. "They see us coming and they get on the radio telling people we're coming."
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Drug smugglers use every possible option to get over the US border including horses, quad bikes and even ultralight aircraft. But the main delivery method is still the oldest, human mules carrying 40lb hessian wrapped bales of marijuana for payment of as little as $500 per trip. It can take them seven days to cross the desert.
The mules, and the thousands of illegal immigrants crossing, strap pieces of carpet to their shoes in an attempt to obscure their footprints. In response the trackers examine thorns for snagged fibres of clothing or hessian. They study the direction of indentations in the soil made by dislodged pebbles. Moisture from a carelessly squashed piece of cactus tells them how far ahead people are. Disturbed soil under a tree reveals how long ago someone stopped to rest, as the shade from the tree moves through the day.
The game of cat and mouse between the Shadow Wolves and the mountain-based spotters goes on daily, and at night, and the tracking methods are having some significant success. They seize an average of 60,000lb of drugs a year with a street value of around $60 million (£38 million). It is impossible to determine how much marijuana, and how many illegal immigrants, get through.