SAN DIEGO » Children walk across the U.S.-Mexico border with crystal methamphetamine strapped to their backs or concealed between notebook pages. Motorists disguise liquid meth in tequila bottles, windshield washer containers and gas tanks.

The smuggling of the drug at land border crossings has jumped in recent years but especially at San Diego’s San Ysidro port of entry, which accounted for more than 40 percent of seizures in fiscal year 2012. That’s more than three times the second-highest — five miles east — and more than five times the third-highest, in Nogales, Ariz.

The spike reflects a shift in production to Mexico after a U.S. crackdown on domestic labs and the Sinaloa cartel’s new hold on the prized Tijuana-San Diego smuggling corridor.

A turf war that gripped Tijuana a few years ago with beheadings and daytime shootouts ended with the cartel coming out on top. The drugs, meanwhile, continue flowing through San Ysidro, the Western hemisphere’s busiest land border crossing with an average of 40,000 cars and 25,000 pedestrians entering daily.

"This is the gem for traffickers," said Gary Hill, assistant special agent in charge of the U.S. Drug Enforcement Administration in San Diego. "It’s the greatest place for these guys to cross because there are so many opportunities."

Customs and Border Protection officers seized 5,566 pounds of methamphetamine at San Ysidro in the 2012 fiscal year, more than double two years earlier, according to Immigration and Customs Enforcement’s Homeland Security Investigations unit. On the entire border, inspectors seized 13,195 pounds, also more than double.

From October 2012 through March, seizures totaled 2,169 pounds at San Ysidro and 1,730 pounds at Otay Mesa, giving San Diego 61 percent of the 6,364 pounds seized at Mexican border crossings. Much of the rest was found in Laredo, Texas; Nogales; and Calexico, Calif.

San Ysidro — unlike other busy border crossings — blends into a sprawl of 18 million people that includes Los Angeles, one of the nation’s top distribution hubs. By contrast, El Paso is more than 600 miles from Dallas on a lonely highway with Border Patrol checkpoints.

Rush-hour comes weekday mornings, with thousands of motorists clogging Tijuana streets to approach 24 U.S.-bound inspection lanes on their way to school or work. Vendors weave between cars, hawking cappuccinos, burritos, newspapers and trinkets.

A $732 million expansion that has created even longer delays may offer an extra incentive for smugglers who bet that inspectors will move people quickly to avoid criticism for hampering commerce and travel, said Joe Garcia, assistant special agent in charge of ICE investigations in San Diego.

Children are caught with methamphetamine strapped to their bodies several times a week — an "alarming increase," according to Garcia. They are typically paid $50 to $200 for each trip, carrying 3 pounds on average.

Drivers, who collect up to $2,000 per trip, conceal methamphetamine in bumpers, batteries, radiators and almost any other crevice imaginable. Packaging is smothered with mustard, baby powder and laundry detergent to fool drug-sniffing dogs.

Crystals are increasingly dissolved in water, especially during the last year, making the drug more difficult to detect in giant X-ray scanners that inspectors order some motorists to drive through. The water is later boiled and often mixed with acetone, a combustible fluid used in paints that yields clear shards of methamphetamine favored by users. The drug often remains in liquid form until reaching its final distribution hub.

The government has expanded X-ray inspections of cars at the border in recent years, but increased production in Mexico and the Sinaloa cartel’s presence are driving the seizures, Garcia said. "This is a new corridor for them," he said.

The U.S. government shut large methamphetamine labs during the last decade as it introduced sharp limits on chemicals used to make the drug, causing production to shift to Mexico.

The U.S. State Department said in March that the Mexican government seized 958 labs under former President Felipe Calderon from 2006 to 2012, compared with 145 under the previous administration. Mexico seized 267 labs last year, up from 227 in 2011.

As production moved to central Mexico, the Sinaloa cartel found opportunity in Tijuana in 2008 when it backed a breakaway faction of the Arellano Felix clan, named for a family that controlled the border smuggling route for two decades. Sinaloa, led by Joaquin "Chapo" Guzman, had long dominated nearby in eastern California and Arizona.

Tijuana registered 844 murders in 2008 in a turf war that horrified residents with castrated bodies hanging from bridges. After the Sinaloa cartel prevailed, the Mexican border city of more than 2 million people returned to relative calm, with 332 murders last year and almost no public displays of brutality.

Alfonzo "Achilles" Arzate and his younger brother Rene, known as "The Frog," have emerged as top Sinaloa operatives in Tijuana — the former known as the brains and the latter as the brawn. The elder Arzate has been mentioned on wire intercepts for drug deals as far as Chicago, Hill said.

He appears to have gained favor with the Sinaloa cartel brass after another cartel operative raided one of his warehouses in October 2010, leading to a shootout and the government seizing 134 tons of marijuana.

Methamphetamine has also turned into a scourge throughout Tijuana, becoming the most common drug offense for dealers and consumers in the last five years, said Miguel Angel Guerrero, coordinator of the Baja California state attorney general’s organized crime unit.

"It has increased a lot in the city because it’s cheaper than cocaine, even cheaper than marijuana," he said.

Disputes among street dealers lead to spurts of violence in Tijuana, said Guerrero, including April’s murder tally of 56 bodies. But the killings pale in numbers and brutality compared to the dark days of 2008 and 2009. While president, Calderon hailed Tijuana as a success story in his war on cartels.

"The Sinaloa cartel, their presence here has been strong enough to the point that no one is pushing back," said the DEA’s Hill. "They just simply want to focus on making money and moving the dope across."