CHICAGO (CBS) — You know the drill when you travel through O’Hare International Airport – take out your shampoo, take off your shoes, put your bag through the X-ray machine, and maybe get a pat-down.

But Dave Savini and our CBS 2 Investigators have discovered that other parts of the airport perimeter are not nearly as secure.

We are not going to reveal our exact location for this report, but all that stands between potential danger and planes at the nation’s busiest airport are chain-link security fences and gates.

Private-contracted security guards and Chicago Department of Aviation security officers are supposed to keep the access points to O’Hare’s airfield, taxiways, and runways safe and secure.

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Our CBS 2 Investigators found gaping holes in that defense.

Day after day, we caught trucks entering O’Hare’s airfield. Nobody inspected them for potential dangers such as weapons, explosives, or possible hidden passengers.

During our stakeout, security guards in yellow vests were seen allowing trucks from all kinds of different private companies right onto the airfield – without even checking driver’s licenses.

One green truck just pulled up, and no one even knew what he was hauling.

“He’s just going to open the gate for him – a bungee cord and that’s it,” Savini said as he observed what was happening at one barbed-wire and chain-link fence gate. “Nobody climbs onto the truck, and no escort in sight.”

Aviation expert Ben Coleman was sitting shotgun with Savini in our undercover van. He is a former Federal Aviation Administration safety inspector and lead investigator with the National Transportation Safety Board.

Coleman was surprised to see guards failing to inspect truck cabs, cargo areas, and undercarriages too. No guards were seen inspecting the green truck at all.

Coleman said special mirrors should be used to inspect the bottom of all vehicles for explosive devices or other dangers.

CBS 2 Investigators never saw this happening at security checkpoints manned by a private contractor – Universal Security.

When asked if there was a serious potential for danger, Coleman said, “The danger is there.”

The Universal Security employees are part of a $91 million contract dating back to 2007. The company provides unarmed guards at the airport.

We found private guards for Universal Security failing to check IDs of contractors. And at other gates, airport employees are breezing through too – we found Universal Security repeatedly failing to swipe access badges before entering secure areas.

“What you have shown me here today raises concern to me,” Coleman said.

One problem may involve training. CBS 2 Investigators obtained this internal report when an unapproved driver was allowed into the secure area. The Universal Security guard who allowed the driver inside said she was never trained to work the gate.

What is supposed to happen at O’Hare’s gates? At another site, Chicago Department of Aviation security officers observe protocols for vehicle inspections.

Badges are checked and swiped and a biometric machine checks fingerprints. On the day CBS 2 came by, the Transportation Security Administration even stopped by seemingly to conduct a random vehicle inspection.

This type of access control is even more important as trucks and private workers descend on O’Hare – all as part of the $8.5 billion O’Hare expansion project.

With so many non-airport workers operating there, rules need to be followed to keep the airport and travelers safe. But that’s not always happening.

Sources tell CBS 2 Investigators there have been repeated failures of truck drivers getting free access to the airfield without being escorted and monitored by badged aviation officials. We saw it happening too, with our own eyes.

“I didn’t see an escort for that truck,” Coleman said as he observed one truck enter the airfield. “I mean, they’re supposed to have an escort.”

In July, two dump trucks from a private construction company were caught in a secure area – their escorts nowhere to be seen.

Coleman said without an escort, a private truck driver has free reign with aircraft taxiing a short distance away.

“They got free reign of where they come and go again. That’s not the desirable situation,” Coleman said. “The whole point of having air traffic control is having control of the traffic not only in the air, but on the ground.”

In September, there was a more serious case. Another dump truck from a different company nearly caused a crash. The driver cut off a taxiing airplane, causing a runway incursion.

His defense was that he said he thought he had time to cross in front of the plane.

“Once you go through those gates or a checkpoint, you are comingled with the operating aircraft – and there’s really nothing to prevent you from either accessing those aircraft or the areas the aircraft operate,” Coleman said.

Coleman said another safety issue exposed by CBS 2 Investigators needs to be dealt with – immediately. It involves a fuel farm attached to the airfield, with giant tanks of jet fuel.

Just by getting through the gate to the fuel farm, Coleman warned that someone could commit “contamination, sabotage, fire, arson.”

He’s especially worried about the pumps where tankers unload their fuel, called transfer pumps.

We showed Coleman how the security gate to the transfer pumps has been left open – repeatedly.

“The gate’s there for a reason with the airport layout or design,” Coleman said.

He added, “If someone were to drive into the transfer station pumps, it could cause a problem a spark and a fire — and you have a major explosion on your hands with hundreds of thousands of gallons of jet fuel.”

When it came to the O’Hare access gates, Coleman’s concluding words were: “It’s not secure. That’s the bottom line. It’s just not secure.”

Chicago Department of Aviation Deputy Communications Director Matt McGrath said in a statement: “Any personnel manning an access point, whether directly employed by CDA or contract security personnel, is expected to comply with our TSA-approved Airport Security Plan. While details of this plan are not disclosed in order to maintain the integrity of our security posture, any deviation from these protocols will be addressed and corrected.”

The TSA is also reviewing our findings.

Meanwhile, Universal Security claims no unauthorized vehicles or personnel were allowed onto the airfield at any time.