(CBS News) When it comes to securing our southern border, at least a few border agents have been stepping OVER THE LINE, ethically speaking, which leaves some Americans living near the border feeling not very secure. Sharyl Attkisson now with our Cover Story along the border between Arizona and Mexico:

John Ladd owns 14,000 wild acres in southern Arizona squared up against the Mexican border. It's land that's become extremely valuable for something besides ranching: For Mexico's illegal immigrants and drug cartels, it's a golden pathway into the U.S.

"The easy part of getting across here is you got three miles to walk, and that's it," said Ladd. "You get picked up at the highway and you're gone."

Five generations of Ladds have lived here. The family journeyed west over gritty trails in covered wagons in 1894.

More than a century later, Ladd watches from his kitchen window as new immigrants -- the illegal kind -- regularly march across his land, oddly undeterred by the border fence, government surveillance cameras, and Border Agents patrolling the property.

When asked how many illegal immigrants he estimates have crossed his property, Ladd replied, "I say about a half a million people have been caught on the ranch. And that's what's been caught - that's not what's got through."

The past few years, the sheer number of Mexicans and those of other nationalities caught on his property IS down.

But Ladd says there's more illegal traffic coming in trucks, filled with drugs. He showed Attkisson where a wall on his property has been cut three times since February: "They sets ramps up on top. They have ramp going into Mexico, have a ramp coming in here. They drive the loaded trucks over the ramps, get 'em in. Come up and cut all of my fences going to the highway."

A recent video from Yuma, Arizona, shows how they do it. The vehicle got stuck with its front end in the U.S., and its tail in Mexico. The passengers ran south.

"The south side is all controlled by the cartel now," said Ladd. "And the cartel has evolved to where they know what they're doing, and there's a lot of money at stake. And Border Patrol is still doing the same thing, deterring and chasing and, you know, we're not deterring anything and we're not doing very well chasing 'em."

Ladd has come up with an astonishing theory as to why the battle seems so futile: He believes some of the federal agents entrusted with policing the border are on the take, working with Mexico's drug cartels.

"There's a lot of people in a lot of positions that can be tempted," Ladd told Attkisson. "I don't think the general public knows how much money is involved with the people trade and the drug trade, and the bribe money to allow it to be coming into the U.S. is astronomical."

More than 40,000 U.S. Customs and Border Protection agents guard the nation's borders, and the vast majority are honest. But drug cartels are working harder than ever to infiltrate their ranks.

"They're using Cold War-style tactics: Money, sex, drugs to convince officers to work with them, and to help get their products and their people across the border," said Special Agent Terry Reed, part of the FBI's ever-expanding operation working to root out corruption.

Reed said that in 2007 there were only six border corruption task forces. Today there are 24.

One of Reed's areas of responsibility is San Ysidro, Calif., at the U.S.-Mexico border, the largest land port of entry in the world. It processes 110,000 people a day. Reed told Attkisson that, just a couple of weeks earlier, an officer has been arrested at the San Ysidro port of entry on corruption charges.

There have been more than just a few bad apples.

A new report from the Department of Homeland Security's Inspector General lists 358 convictions of Customs and Border Protection employees and their associates since 2004.

Complaints of misconduct are up by 77 percent.

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