NOGALES, Ariz. - Hawks circle above the lines of traffic at the hot, arid border crossing into Mexico. Sagebrush catches clothes tossed by fence climbers. Three curious, dusty horses watch the federal agents tapping on car windows, opening trunks, looking in vain for contraband.

An agent notices the horses and wonders aloud if they're wild. A colleague notes the temperature: 92 degrees.

"We're sucking up a lot of exhaust out here," supervisory Customs and Border Protection officer Edith Serrano says, shrugging in her uniform.

This is what the Obama administration's new commitment to help Mexico fight its drug cartels looks like.

President Barack Obama this spring promised his Mexican counterpart, Felipe Calderon, that the United States would fight two of the biggest contributions U.S. residents make to the drug cartels Calderon has vowed to eradicate: cash and weapons, the latter hard to come by in Mexico.

For the past five weeks, hundreds of agents participating in a newly intensified $95 million outbound inspection program have been stepping into southbound traffic lanes, stopping suspicious-looking cars and trucks.

The Associated Press fanned out to the busiest crossings along the Mexican border - San Diego, Nogales, El Paso and Laredo - to see how effective the inspections are.

The findings? Wads of U.S. currency headed for Mexico, wedged into car doors, stuffed under mattresses, taped onto torsos, were sniffed out by dogs, seized by agents and locked away for possible investigations. No guns were found as the reporters watched; they rarely are.

"I do not believe we can even make a dent in (southbound smuggling) because that assumes the cartels are complete idiots, which they're not. Why in the world would they try to smuggle weapons and currency through a checkpoint when there are so many other options?" said Border Patrol Agent T.J. Bonner, president of the agents' union.

According to CBP, from March 12 to April 30 officers seized:

Fifty-one pieces of ammunition, weapons parts and guns, a minuscule fraction of the 2,000 weapons the Mexican government estimates are smuggled south every day.

$12 million in cash, less than one-tenth of 1 percent of the $17 billion to $39 billion the U.S. Justice Department estimates is illegally sent to Mexico from the U.S. annually, but more than the $10 million seized in outbound checks in 2008.

Sixty-one people on charges involving weapons or currency offenses and on outstanding warrants.

Millions of cars pass into Mexico from the United States every year. The federal government doesn't keep track, but a count by Texas A&M International University's Texas Center for Border Economic and Enterprise Development shows more than 27 million vehicles a year drove into Mexico just from Texas.

The outbound checkpoints the AP observed stopped sometimes one out of four cars, sometimes one out of 100, and not every day. Even that amount created huge traffic backups at some locations and, agents said, might have allowed spies to call any smugglers heading that way and warn them to put off their Mexico trip.

Agents across the border said the first few minutes of their operation are the most precious. That's how long it takes for "scouts" watching from a bridge in San Diego lined with taxis to radio ahead to smugglers to stay away. In Nogales, a dozen men dashed along a Mexican hill about 150 yards from the checkpoint last week.

"We tend to see spotters up there," said CBP agent Brian Levin. "They sit up on those hills and watch everything we do."

Inspectors retreat, then mount another "surge" after a while standing on the side of the freeway.

Some of those stopped were sanguine, others annoyed.

"I guess they think I have drugs or something," said Daniel Saucedo, a 15-year-old Albuquerque high school student who clambered out of the passenger side of a small white pickup truck with his two dogs last week in El Paso. "It's dumb," he said.

William Molaski, port director in El Paso, said agents at his four El Paso bridges haven't found much since the focus on outbound checks started in early April - one handgun and only about $400,000 - "but not for lack of trying."

Without providing any numbers, Homeland Security Secretary Janet Napolitano told attendees at the Border Trade Alliance International Conference on April 21 that, just a few weeks into the intensified outbound inspections, she was amazed at how much had already been seized. "It's unbelievable," she said. "So the notion that there wasn't a river of cash and a flood of guns going into Mexico is a myth. I mean, there was. We want to stop that river."

CBP's 2010 budget request, released May 7, includes an additional $46 million specifically targeted at southbound enforcement.

Customs inspectors' techniques range from primitive to high-tech, with about an equal success rate. Sometimes a small white truck drives slowly alongside vehicles that have been pulled over, beaming X-rays at them to reveal hidden cash or weapons. A smaller X-ray unit scans spare tires or pieces of luggage, a hand-held density meter called a "Buster" can reveal hidden compartments loaded with cash, a fiber-optic scope snaked into gas tanks looks for hidden cargo and trained dogs can sniff out cash or weapons.

But before they get to any of the gadgets, officers knock with a knuckle or flat palm on a car's body panels. And they ask, again and again: "Do you have any weapons? Cash? Merchandise?"

Often the dogs make the finds.

Grill, a "currency canine," smelled something on 63-year-old Isabel Ortega Garcia on April 3 in Hidalgo, Texas, when Ortega was walking into Mexico. When Grill got excited, agents patted Ortega down and found $148,000 in neat wads of $100 bills taped around her waist.

Two weeks earlier in Laredo, Akim sniffed cash under the floor of a southbound bus. Under the seats, in a hidden compartment, were 75 bundles of bills totaling $2,997,510.

But even finding that much cash doesn't always yield an arrest. Without a U.S. attorney's say-so, the best an agent can do is seize any cash amounts over $10,000 that the traveler does not declare, hand them a receipt and send them on south.

The best case scenario for agents who seize undeclared currency is that federal prosecutors decide to bring charges and begin a forfeiture procedure. But often it is a race against the clock as inspectors on the scene try to collect enough evidence to make it an attractive case for prosecutors.

Although Laredo leads the country in cash seizures right now, even there seizing cash is rare, and arresting someone even more unusual, a weapons seizure rarer still. And that's where the inconvenience to travelers and agents' frustration set in.

Officers have no booths, no signs for drivers or lanes to pull people over in. Yet.

"We don't have the infrastructure that we need to conduct safe outbound inspections," said Oscar Preciado, director of San Diego's San Ysidro port of entry.

The Obama administration has budgeted $269 million to upgrade these southern ports, adding lanes and pull-over spaces. Perhaps most importantly they'll be adding shade for the wilting agents who wade into traffic under the blazing sun.

Over five hours on a recent day, outbound traffic from Laredo, Texas, to Nuevo Laredo, Mexico, experienced the gamut from clear, moving traffic, to multiagency teams laying travelers' lives bare, unpacking, X-raying and interviewing.

Mexican customs inspector Ricardo Briseno, 27, says the increase in U.S. inspections of Mexico-bound cars has made his job easier, even though the only effective solution would be to stop every car.

"At least it's something," he said. "We are working together on a shared problem."