NACO, Ariz. -- The immigrants jumped the border fence and darted behind heavy brush, unnoticed by US Border Patrol agents. But miles away, in an air-conditioned control room, their images appeared on a video screen.
A National Guard soldier watching the monitor radioed an agent and guided him through a vast desert expanse, telling him to stop his vehicle on a narrow road.
"They're in the bushes," he told the agent, who and arrested about a half-dozen immigrants.
Surveillance cameras that can peer miles into Mexico have become a principal tool to defend the nation's 2,000-mile-long border.
Since the late 1990s, camera towers have been erected in many populated border areas, from Calexico in California's Imperial Valley to Brownsville, Texas. Next month, of.cials in the San Diego area are scheduled to start camera operations on a volatile 6-mile stretch of border across from Tijuana.
Arizona's experience with the cameras, however, illustrates the squeeze-and-bulge phenomenon of illegal immigration. When the Border Patrol seals parts of the frontier with fences, technology and additional agents, illegal immigration moves elsewhere.
The cameras helped to produce a dramatic drop in illegal crossings in some border towns, including Naco, Nogales, and Douglas in southeastern Arizona.
But the .ow across the state's border has not slowed. Much of it merely shifted west to the Yuma area, the site President Bush chose to highlight his plans to beef up border security and to admit that the United States did "not yet have full control of the border."
The region is now one of the busiest illegal immigration corridors in the nation.
Apprehensions of immigrants have increased 75 percent since 2001. Assaults against agents also have surged. Smugglers are known to throw rocks at hovering helicopters.
In San Luis, a small border town 20 miles south of Yuma, immigrants swarm across the border in wild runs that overwhelm agents.
Bush said last week that he would send 6,000 National Guard troops to assist at the border.
In addition, both houses of Congress have proposed fortifying hundreds of miles of the frontier with fences and more high-technology tools.
The prospect of a beefed-up border is welcomed by most lawenforcement of.cials and politicians in this state; concentrated efforts, they say, have made a difference.
But because of the shifting nature of illegal immigration, many are not convinced that more cameras and fences will reduce the .ow.
"It hasn't gotten any better," said Tony Estrada, the sheriff in Santa Cruz County in southeastern Arizona. "They sealed up the urban areas, but now the activity is in the rural areas."
Five years ago, the busiest border- crossing route in Arizona went through Douglas, a dusty town lying on a high plain in the southeast corner.
Day and night, hordes of immigrants jumped the rusty fence and ran into alleys and neighborhoods.
"They even knocked on my door," Mayor Ray Borane said.
"They would want water or want to use the phone."
Today, the second-biggest Border Patrol station in the country houses more than 500 agents, and sits on the highway into town.
In addition, more than a dozen towers equipped with remote video surveillance cameras rise 60 feet above the streets and surrounding desert. The number of apprehensions in the area patrolled by the station has dropped from 262,000 in 2000 to 71,000 last year.
Border buildups in Naco and Nogales have cut migration .ows through those southeastern cities.
Many agents and local law-enforcement of.cials credit additional staf.ng and high-technology tools.
The cameras can keep an eye on dozens of miles of border.
Equipped with infrared capabilities, they can spot illegal crossers even at night. Thermal imaging outlines their bodies behind bushes and other vegetation. The cameras can zoom in on people climbing trails 5 miles away.
Operators, often National Guard troops, scan a half-mile stretch of border in seconds, using a control stick. A push of a button switches the screen to another stretch of border.
The smugglers and immigrants know they are being watched; sometimes they wave and make rude gestures at the cameras. If crossings are not noticed by camera operators, motion sensors can alert them.