US federal officials say the FBI is probing whether a man tried to ignite his shoes on a Washington DC to Denver flight.
An official said an air marshal on the flight apparently restrained him.
F-16 jets were scrambled to intercept the plane and escort it to land safely at Denver airport
Kristin Lee of the Transportation Security Administration said the man is being interviewed by law enforcement in Denver.
Two officials said the man is a Qatari diplomat who has been in the US for years.
Unlike the Christmas Day bomb attempt on an Detroit-bound airliner, officials say it is not immediately clear what the man was trying to do.
Officials insisted it was still too early to tell whether the incident was an attempted act of terrorism or a misunderstanding.
The US Transportation Security Administration said: "The TSA is monitoring an incident on board United Airlines flight 663 from [Washington to Denver] after receiving initial reports that a Federal Air Marshal responded to a passenger possibly causing a disturbance on board this aircraft."