UPDATE: "JTTF investigated, and the investigation shows that this was a non-serious incident and all is clear at this point," according to Sandra Berchtold.
Berchtold, a spokeswoman for the Joint Terrorism Task Force in Detroit, said Sunday's alert was caused by a passenger who "spent a lengthy time in the restroom."
"This raised concerns, so an alert was raised," she said.
The passenger spent about an hour in the bathroom and got upset when he was questioned by the crew of the flight from Amsterdam, Netherlands, according to government sources. Law enforcement agents questioned the man Sunday.
The man was detained for unruly behavior Sunday on Flight 253 from Amsterdam to Detroit, one day after Umar Farouk Abdulmutallab was charged with attempting to blow up the same flight in a separate event, widely reported Friday.
A Delta spokeswoman said all 257 passengers were safely taken off the plane. Delta operates the Northwest flight.
"The flight landed safely at Detroit International Airport at approximately 12:35pm est without incident," according to a statement from the Transportation Security Administration. "The aircraft has been moved to a remote location for additional screening. TSA and law enforcement met the aircraft upon arrival, the passenger is now in custody."
Sue Elliott, a Delta spokesperson, told CBS News the passenger in custody "was verbally disruptive. Out of abundance of caution the flight crew asked that the plane be met on arrival by law enforcement."
According to reports, passengers and crew became suspicious of the man for some reason while he was locked in the bathroom. There is no indication he attempted to harm anyone aboard the plane or sabotage the Boeing 737 in any way.
Security has been ratcheted up on international flights into the United States since the incident Friday.