A passenger was subdued by a federal air marshal during an incident aboard an airliner flying from Washington to Denver on Wednesday night, sources told NBC News.
The FBI was investigating whether the man, a Qatari diplomat, attempted to ignite something aboard the plane.
United Flight 663, a Boeing 757 with 157 passengers and six crew members aboard, landed safely at Denver International Airport, airline and airport officials said.
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A half hour before landing, an air marshal smelled smoke and confronted a man emerging from a bathroom who claimed he was trying to set his shoes on fire, WNBC reported.
It was uncertain if anything actually was on fire or if this was a joke, NBC reported. Investigators were looking to see what might be in the man's shoes, but sources told NBC that it appeared that there were no explosives.
Sources told NBC News that the passenger who was subdued was a Qatari diplomat who would have full diplomatic immunity.
After the pilot declared an emergency, military jets scrambled and escorted the jet into Denver, where the plane remains in a secure area, NBC affiliate WRC reported.
The incident follows an attempted bombing on Christmas Day by a Nigerian man who tried to detonate explosives hidden in his underwear on a flight to Detroit from Amsterdam. The susptect, Umar Farouk Abdulmutallab, was burned in the incident, which was foiled by passengers.
The Christmas attempt led to mandatory screening of air travelers from 14 countries. Qatari is not among the 14.
Last week, the United States announced new security measures to replace the mandatory screening.
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The 14 countries were those on the U.S. list of "state sponsors of terrorism" — Cuba, Iran, Sudan and Syria — as well as Afghanistan, Algeria, Iraq, Lebanon, Libya, Nigeria, Pakistan, Saudi Arabia, Somalia and Yemen.
Saudi Arabia, Algeria and Nigeria — U.S. partners in the fight against al Qaida — were angered at being on the list.