"Von Walter said the first incident occurred about 7:30 a.m. when a man was seen running up a down escalator between terminals 7 and 8, operated by United and Continental airlines. When security officers failed to find the man inside the terminal area, airport and TSA officials closed down and evacuated terminals 6, 7 and 8, which are interconnected."
This looks like the early days of network-spread computer viruses, when MIS would "shut down" everything until the "situation is under control."
