According to officials, police met a plane that landed Sunday at Detroit Metro Airport.
Officials said the pilot of Frontier Airlines flight 623 from Denver to Detroit alerted the airport tower of an emergency on board.
The plane landed safely and was met by police. Three passengers who were making multiple trips to the bathroom were taken into custody once the plane landed.
The three passengers were released Sunday without charges.
Passengers got off the plane and boarded buses. They were taken to a separate secure location where they were questioned.
Police determined the situation was not a serious threat, but were investigating.
Flight 623 left San Diego at 6:30 a.m. Sunday. It stopped over in Denver before finally landing in Detroit at 4:22 p.m.
In Denver, the FBI said that the North American Aerospace Defense Command scrambled F-16 fighter jets to shadow the plane "out of an abundance of caution." The plane was searched and nothing was found, the FBI said.
Two F-16s were dispatched to shadow the Airbus, said John Cornelio, spokesman for the North American Aerospace Defense Command, which is headquartered at Peterson Air Force Base in Colorado Springs.
Detroit Metropolitan Airport spokesman Scott Wintner says the Frontier flight crew radioed to request police help when the plane landed, prompting responders to greet the flight and question passengers after the aircraft taxied to a remote location at the airport.
There was concern about a second plane -- but after much investigation, officials have determined it's not a threat.