In less than three minutes, an intruder exploited a security gap at one of the nation’s busiest airports and stepped into the path of an airplane hurtling down a Colorado runway with 231 people aboard. The 41-year-old man slipped unnoticed past motion detectors in a remote corner of Denver International Airport, which sprawls across open plains and covers an area twice the size of Manhattan. He quickly scaled an 8-foot perimeter fence topped with barbed wire, then walked unobstructed onto he...