Several hundred times over the last decade, intruders have hopped fences, slipped past guardhouses, crashed their cars through gates or otherwise breached perimeter security at the nation’s busiest airports – sometimes even managing to climb aboard jets.

One man tossed his bike over a fence and pedaled across a runway at Chicago O’Hare, stopping to knock on a terminal door. Another rammed a sports-utility vehicle through a security gate at Philadelphia International and sped down a runway as a plane was about to land.

At Los Angeles International, a mentally ill man hopped the fence eight times in less than a year – twice reaching stairs that led to jets.

An Associated Press investigation found 268 perimeter breaches since 2004 at airports that together handle three-quarters of U.S. commercial passenger traffic. And that’s an undercount, because two airports among the 31 that AP surveyed didn’t have data for all years. Boston’s Logan and the New York City area’s three main airports refused to release any information, citing security concerns.

Until now, few of the incidents have been publicly reported. Most involved intruders who wanted to take a shortcut, were lost, disoriented, drunk or mentally unstable but seemingly harmless. A few had knives, and another was caught with a loaded handgun.

None of the incidents involved a terrorist plot, according to airport officials.

The lapses nevertheless highlight gaps in airport security in a post-9/11 world where passengers inside terminals face rigorous screening and even unsuccessful plots – such as the would-be shoe bomber – have prompted new procedures.

“This might be the next vulnerable area for terrorists as it becomes harder to get the bomb on the plane through the checkpoint,” said airport security expert Jeff Price.

Since the Sept. 11 attacks, hundreds of millions of dollars have been spent to upgrade perimeter fencing, cameras and detection technology. Many airports have dozens of miles of fencing, but not all of that is frequently patrolled or always in view of security cameras.

Airport officials declined to outline specific measures, other than to say they have layers that include fences, cameras and patrols. Authorities said it is neither financially nor physically feasible to keep all intruders out.

“There is nothing that can’t be penetrated,” said LAX Police Chief Patrick Gannon, noting that even the White House has struggled with fence jumpers.

The AP’s analysis was prompted by a breach last spring in which a 15-year-old boy climbed a fence at Mineta San Jose International Airport, hoisted himself into a jet’s wheel well and survived an almost six-hour flight to Hawaii. .

Afterward, an airport spokeswoman said breaches are more common than people realize.