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.

In all, an Associated Press investigation has found 268 perimeter security breaches between January 2004 and January 2015 at more than two dozen of the busiest U.S. airports that together handle three-quarters of commercial passenger traffic.

Incidents included fence jumpers taking shortcuts, intoxicated drivers crashing through barriers and mentally ill intruders looking to hop flights. None was terrorism-related.

Airports say breaches are relatively rare and that it is neither financially nor physically feasible to keep all intruders out. Security includes fences, cameras and patrols — but there are gaps.