US federal air marshals subdued a Qatari diplomat after an disturbance aboard a flight from Washington to Denver late Wednesday that fueled suspicion he was trying to detonate a shoe bomb, US media reported.
But it remained uncertain whether the passenger was joking or anything was actually set on fire. There were no reports of an explosion on the plane.
The FBI was investigating whether the man, identified by ABC News as Qatari diplomat Mohammed al-Modadi, tried to ignite something aboard the plane. NBC News said bomb-sniffing dogs found no traces of explosives aboard the aircraft.
NBC said that a half hour before United Airlines flight 663 was due to land in Denver carrying 157 passengers and six crew, an air marshal smelled smoke and confronted a man who had spent an extended period of time in the bathroom and claimed he was trying to set his shoes on fire.
Al-Modadi has full diplomatic immunity as the third secretary and vice-consul of the Qatari embassy in Washington.
He was placed in custody, the Transportation Security Administration (TSA) said.
The flight from Washington's Reagan National Airport landed safely in Denver following the disturbance.
After the pilot declared an emergency, two F-16 fighter jets intercepted the aircraft around 6:45 pm (0045 GMT Thursday) under the authority of the North American Aerospace Defense Command (NORAD).
"Shortly before landing in Denver, a passenger possibly caused a disturbance on the plane. Upon intercepting the aircraft, the F-16s escorted the aircraft until it landed safely without incident at approximately 6:50 pm (0050 GMT) where the plane was met by local law enforcement," NORAD said.
TSA said it was "monitoring" the incident "after receiving initial reports that a federal air marshal responded to a passenger possibly causing a disturbance on board this aircraft."
The security agency said, "All steps are being taken to ensure the safety of the traveling public."
The incident came a week after the United States unveiled new security measures subjecting all US-bound plane passengers to screening methods that use real-time intelligence to target potential threats, replacing the mandatory screening of passengers from a blacklist of 14 mainly Muslim countries that had angered some allies.
The measures were announced in the wake of a Nigerian man's failed attempt to detonate explosives concealed in his underwear on a Detroit-bound flight from Amsterdam on Christmas Day.