-- The two Americans in the blue-gray Ford F-150 were slow to make eye contact, and the veins in the driver's neck began to bulge slightly as he explained what the pair had been doing in Tijuana.

Pointing his flashlight under the truck, U.S. customs inspector Edric Onsgioco could see the bolts around the gas tanks had been loosened. Then Sasha, a drug-sniffing Belgian shepherd, let out a howl and made a beeline for the vehicle.

Pulling apart the truck's underbelly, inspectors found two bread box-sized cavities in the gas tanks -- but no drugs inside.

"This truck has been used for smuggling, probably heroin or coke. But this was a dry run," Onsgioco said. "The smugglers are being careful; they are testing us. But you know they'll be back with full tanks."

Authorities had to let the pair go, but made them hire a tow truck to haul off the pickup. Onsgioco said the pickup was not legal for street use because holes had been cut in the top of the gas tanks for access to the hidden cavities. Fuel could slosh out and ignite, he said.

America's efforts to make it tougher for illegal immigrants, drug traffickers and terrorists to cross even remote parts of the border have brought smugglers back to formal border crossings, where more cars, trucks and big rigs crammed with drugs are trying to slip across.

Sneaking drugs through the most-watched crossing points on a border better fortified since the Sept. 11, 2001, terrorist attacks, and now war in Iraq, is an indication that smugglers are trying to transport more than custom inspectors can detect, U.S. and Mexican officials say. For every load caught, countless others slip through undetected.

"Anti-terrorism efforts have pushed most of the loads back to us, back to the border crossings all over," said Oscar Preciado, customs port director at San Diego's San Ysidro entry point, the world's busiest land border crossing.

Heightened security in the first weeks after the Sept. 11 attacks saw drug seizure quantities plummet along the border. But customs inspectors at the 40 official road crossings into Arizona, California, New Mexico and Texas have been busier lately.

Inspectors in South Texas set a record for their region in fiscal 2002 by seizing 385,777 pounds of cocaine, heroin and marijuana. West Texas and New Mexico border crossings also set a record in the period that ended Sept. 30, capturing 326,371 pounds of narcotics.

"The smugglers are becoming more brazen," said Rick Pauza, a customs spokesman for the border crossing between Laredo, Tex., and Nuevo Laredo, Mexico. "There's such a large quantity of narcotics that they're trying to move through now that they are willing to lose substantial quantities if we find them."

In California, drug seizures were down 30 percent last fiscal year, to 315,442 pounds. But Preciado said his inspectors are encountering more test runs like the one Onsgioco discovered, and the officers who do make busts are finding larger loads.

"The smugglers aren't afraid to come at us," said Preciado, whose office looks out on 24 lanes of cars waiting to cross into San Diego.

Authorities say more drug gangs are shipping Colombian cocaine to Mexico rather than flying it directly to the United States.

Until the late 1990s, Colombian gangs flew cocaine into U.S. airports or airdropped it near the U.S. border for Mexican smugglers who used vans, donkeys and human couriers to carry it farther north. Mexico's Juarez cartel used Boeing 727s to fly drugs as far north as Manhattan.

But Mexican and U.S. authorities improved radar surveillance -- a trend reinforced in the United States after the Sept. 11 attacks. The U.S. Border Patrol also increased surveillance of the vast terrain between formal border crossings.

"September 11 has made it harder to cross contraband into the United States," said Charles Harrison, special agent in charge of customs operations in South Texas. "We have dramatically increased the pressure all over the border."

The U.S. Drug Enforcement Administration estimates 75 percent of the cocaine that reaches the western United States is shipped from South America to Mexican harbors stretching from the resort of Puerto Vallarta to Tijuana and then moved north.

Antonio Martinez, attorney general for Mexico's Baja California state, said efforts to close secret landing strips as well as crack down on crooked cops who protect drug loads have hindered smuggling.

"But anyone who says we don't have problems with drug smuggling, with organized crime and with the violence these elements bring is living on another planet," he said.