By Ian Bush, Justin Finch

PHILADELPHIA (CBS) – Security could hardly be tighter inside Philadelphia International Airport, but what about outside? A new investigation finds hundreds of perimeter breaches over the past decade at the country’s busiest airports, and PHL is nearly at the top of the list.

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There was the guy who crashed his Jeep through a fence and sped down the runway, forcing dozens of jetliners to circle the airport before he was chased down. That was three years ago. Back in 2006, an older woman drove into a secure area, thinking it was Sears at a mall.

An Associated Press investigation has found 25 perimeter security beaches at PHL over the past 11 years. Only San Francisco International has had more, among more than two dozen of the country’s busiest airports.

Aviation expert Arthur Wolk says a single security breach is one too many:

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“I don’t think it matters if it is intentional or not. The one intentional person could have been a terrorist that generates a horrible disaster.”

The study says none of the breaches were terror related.

PHL officials say out of all the intrusions, only the Jeep driver’s was deliberate. Over the past couple years, there have been only two incidents — improvements the airport chalks up to increased patrols, extra cameras, and the installation of more guard rails and crash beams.

“We did have extensive and expensive perimeter hardening that went into effect,” says airport spokeswoman Mary Flannery.

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The AP study spans back to 2004.