The young boy made it past the security officials at the Minneapolis-St Paul International Airport and flew to Las Vegas alone last week, an airport official said.
The boy was screened at security during the initial leg of his trip, said airport spokesman Patrick Hogan.
Security camera tapes showed that the child then managed to board a Delta flight undetected, Hogan said.
The boy apparently told a flight attendant that his parents were sitting in the back of the plane, a source close to the investigation told NBC News.
Officials then took the boy into custody and were arranging his return to Minneapolis, the report said.