ALTAR VALLEY, Ariz (Reuters) - U.S. Border Patrol agent Galen Huffman leans over the saddle to look at faint tracks in a cattle trail leading up from the Mexico border.
He follows the tracks at a brisk trot through thick brush, up and down into rocky washes, then pauses as his horse twitches its ears and turns around nervously.
"I have bodies," he says, as the first of a group of 11 illegal immigrants, wrapped in hats and scarves against the chill morning air, peel themselves up from the desert floor several miles from the nearest road.
Horses have been part of the Border Patrol since the agency was founded to secure the United States borders against liquor smugglers and unlawful immigrants in the 1920s, and now they are making a comeback.
Agents dressed in leather chaps and broad-brimmed hats are increasingly being used to regain control over the most rugged areas of the southwest frontier with Mexico and now on the northern border with Canada.
"Most of the traffic is being pushed into these mountainous areas which are harder to work. They are very remote," said Bobbi Schad, a horse patrol supervisor from Tucson. "With a horse you can get up in there."
"They realized we were so much better at controlling certain areas, so they said 'hey, let's keep utilizing an old-school tool and go back to the basics."'
When the Border Patrol was founded in 1924, recruits on the southwest border had to provide their own horses, tack and guns, although they were given bullets, animal feed and $25 a week.
They were sent out on the back trails from California to south Texas to tackle "tequileros" -- smugglers hauling barrels and goatskins filled with liquor north from Mexico on horses, mules and burros during the period of Prohibition.
Modern-day mounted agents secure many of the same out-of-the-way trails as their predecessors, although now they track groups of illegal immigrants and hardy drug traffickers, some armed with knives, pistols and assault rifles.
Like their forebears, present-day Border Patrol agents continue to use sturdy quarter horses -- so called as they are the fastest breed over a quarter mile, and are also renowned for their strength and stamina.
Most of the stocky, muscular animals are bought from horse dealers or at auctions, although a number are seized from Mexican smugglers caught riding over the border with loads of marijuana crammed into burlap sacks.
"Some of the smugglers have a good eye for a horse," Schad told Reuters as she rode out over a sandy cattle trail in the Altar Valley south of Tucson.
"They are raised in the mountains and they are good ranch horses ... and they are some of the best that we have."
Last year, more than 870,000 illegal immigrants were arrested crossing over the border from Mexico, more than a third of them through the wilds of Arizona.
In addition to getting agents swiftly and stealthily up into the rugged, mountainous areas through which migrants and smugglers increasingly cross, agents say their horses are also active partners in detecting border crossers.
"Their sight and hearing is much sharper than ours. When they sense someone ... you feel it, and you know to get ready," Huffman said.
Mounted units in the busy smuggling corridor south of Tucson frequently arrest groups of 15 or 20 illegal immigrants, and sometimes more than 100 a time. Agents say their horses' imposing size gives them a good view over the uneven terrain and helps them establish control.
Once they have apprehended a group, agents often have to lead detainees for a mile or more to reach ranch roads where they can hand them over to colleagues with a vehicle. It is there that the horses' stock management skills come into their own, agents say.
"It's what we call cow sense. It's a herding instinct," Schad told Reuters, as she trailed a group of 18 Mexican migrants out of the desert on her horse, Freckles.
"They gather people up ... and if they run, they have the ability and desire to chase them down," she added.
As the Border Patrol rediscovers the special abilities that horses have, mounted units are starting to play a wider role in border policing.
In the Tucson sector -- which is the busiest along the 2,000-mile (3,200-Km) southwest border -- the patrol now has around 150 horses and 87 trained riders, around four times the number that they had five years ago.
Horse units are also being used in Texas and California -- where they played a key role saving livestock from the raging wildfires near San Diego last October -- as well as in Spokane, Washington, where the Border Patrol recently started using mustangs to secure a section of the Canadian frontier.
While a drift toward hi-tech in border policing gathers pace, with unmanned spy planes, ground radar and an array of electronic sensors, mounted agents are confident their horses will continue to play a key role in years to come.
"They are a good tool for going out and 'cutting for sign' in areas that the hi-tech stuff won't reach," Huffman said, using a term for tracking, at the end of two days in which he and his colleagues stopped 29 undocumented immigrants crossing from Mexico. "They still have a good future."
Reporting by Tim Gaynor; Editing by Eddie Evans