With airline passengers fuming about long security lines inside terminals, an Associated Press investigation finds that outside the nation's busiest airports someone gets over fences or through vehicle gates that guard planes and runways about every 10 days.

Despite investments to fortify some airfields, the incidents remain as frequent as ever.In 2015, a woman walked onto the tarmac in San Francisco and tried to flag down a jet. John F. Kennedy International in New York City had three fence jumpers.Altogether, AP learned of 345 perimeter security breaches at 31 major U.S. airports since 2004. And that's an undercount, because some authorities tried to suppress information.Airport officials say no intrusion has been linked to terrorism and that most people are caught quickly.