SAN YSIDRO — On a recent gray morning, federal inspectors waded into the sea of border traffic heading to California and began pulling over cars and opening doors as another wave of frenzied searches hit.
In the back of a Nissan Pathfinder, two trembling women and a man were found hiding under a blanket. Moments later, four young men from Jalisco spilled from a green van. Popping the trunk of a Neon, inspectors discovered two adult brothers and their sister packed inside.
Within 1 1/2 hours, the agents had arrested 25 suspected illegal migrants, part of a recent surge in car smuggling cases that reflects a shift in the flow of illegal immigration. Immigrants and smugglers, experts believe, are adapting to crackdowns on document fraud and increased enforcement in Arizona, the busiest illegal immigration corridor.
Sensing a soft spot in border enforcement, smugglers appear to be playing the odds, hoping to slip cars loaded with illegal immigrants through during the crush of daily traffic.
"It's a numbers game," said David Salazar, a chief Customs and Border Protection officer at the port of entry.
The surge began last year when nearly 50,000 illegal immigrants -- the majority concealed in trunks or compartments -- were caught in vehicles traveling from Mexico to California. It was nearly double the figure from 2003.
At the San Ysidro port -- the busiest and largest of California's five land crossings -- officials estimate that the number of people caught concealed in cars last year had tripled since 2000, from 10,600 to about 30,000. And the pace has continued this year, officials say.
Smugglers still use inventive ploys. People come stuffed in hollowed-out dashboards and gasoline tanks. They swelter in "coffin compartments" near engines and grinding drive shafts. Others dangle on plywood planks beneath moving vehicles. And a little girl last year was found stuffed into a pinata.
Some try to pass inconspicuously as U.S. citizens or visa holders by presenting false documents.
But most simply squeeze into car trunks, curling next to other migrants -- sometimes people they don't know -- and often wait hours in the darkness and heat. Inspectors earlier this year found 16 people piled atop one another pyramid-style in a van.
Most illegal immigrants apprehended are voluntarily returned to Mexico.
"It all comes down to desperation, and the smugglers -- being as depraved as they are -- they prey on people's desperation and human misery," said Adele Fasano, Southern California director of field operations for the U.S. Customs and Border Protection agency.
Ten years ago, about 80% of illegal immigrants apprehended at the San Ysidro crossing tried walking through in the pedestrian aisles, many using fraudulent documents. As inspectors became more adept at detecting fraud, the pattern changed. Now, 80% of apprehensions are car-smuggling cases, according to Fasano.
"Not much is getting past us in the pedestrian area. As a result, the smugglers shift their focus to the area where they see they have a higher level of success," she said.
About 75 inspectors fan out to meet the crush of 45,000 to 60,000 cars that cross daily. Salazar estimates that about 1% -- as many as 600 cars -- are transporting drugs or illegal immigrants.
At the border, customs officers scan drivers' faces looking for telltale signs: A sweaty brow on a cold day or an overly friendly demeanor, they say, can give them away.
Still, searching every vehicle is impossible when many drivers are regular commuters, the employees and shoppers at San Diego-area businesses that represent an integral part of the border economy.
"The law enforcement side in me ... says let's check every car out there," Salazar said. "But we have to balance the economic interests and law enforcement interests."
Watching inspectors scramble from one vehicle to another, Salazar said, "It makes you wonder what else is out there."
Smuggling networks range from sophisticated operators who charge as much as $2,000 a head to be brought in by an American citizen in a late-model car (some smugglers have used Hummers) to discount groups that charge $300 to be transported in the trunk of an old junk car driven by an illegal immigrant.
Finding drivers, authorities say, isn't a problem. Smuggling rings, dangling the prospect of easy money, recruit homeless people and down-on-their-luck gamblers at casinos. So many San Diego-area teenagers are involved -- they make as much as $500 per crossing -- that federal agents give talks at high schools warning students of the risks.
Convicted noncitizen smugglers often face prison terms ranging from 1 1/2 to 10 years. But because of limited resources, authorities prosecute relatively few. They focus on repeat offenders and those who endanger lives during crossings.
