Airport spokesman Jeff Lea said the man -- who was discovered sleeping during a routine check of the jet at about 6:40 a.m. Wednesday -- apparently walked through the exit side of a vehicle checkpoint while officers staffing the checkpoint were busy on the entrance side, the St. Louis Post-Dispatch reported Thursday.
"There was a breakdown in this layer of security," Airport Police Chief Paul Mason said.
Mason said the man told investigators he was at the airport to try to find a truck driver willing to give him a ride to Chicago and ducked inside the jet to escape from the rain.
The police chief said the man, who officers planned to charge with trespassing, did not have any sinister motives for sneaking onto the jet.
"This guy had no intention to do anything wrong," Mason said.