Breaking story from Denver: ABC News reports, "Federal air marshals subdued a man who authorities say attempted to 'light his shoes on fire' on a United flight from Washington Reagan to Denver Wednesday night, federal law enforcement officials told ABC News." However, it's unclear if there were explosives— NBC News says that an air marshal smelled smoke and confronted the man, who said he was trying to light his shoes: "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 his shoes."

What's more, the person in custody "was identified by authorities as a diplomat in the Qatar embassy in Washington, Mohammed al Modadi. The FBI said the man had full diplomat immunity as the 3rd secretary and vice-consul," according to ABC News.

Update 10:50 p.m.: NBC News' Pete Williams says, based on information so far, it seems like the diplomat was smoking and made an inappropriate remark in response to the air marshal's inquiry. And investigators reportedly did not find explosives in the shoes.

The plane landed safely—but it was guided to Denver International with F16 fighter planes.

Update 11:30 p.m. : A US security official tells ABC News "it may have been a massive misunderstanding" and the diplomat might have been "sarcastic" when he allegedly said, "I'm lighting my shoes on fire."