A Qatari diplomat trying to sneak a smoke in an airplane toilet sparked a bomb scare on a flight from Washington to Denver, with fighter jets scrambled and law enforcement put on high alert, officials said.
No explosives were found on the man, and officials do not believe he was trying to harm anyone, according to a senior law enforcement officials who spoke on condition of anonymity.
An Arab diplomat briefed on the matter identified the diplomat as Mohammed Al-Madadi.
Two law enforcement officials said investigators were told the man was asked about the smell of smoke in the toilet and he made a joke that he had been trying to light his shoes - an apparent reference to the 2001 so-called "shoe bomber" Richard Reid.
Officials said air marshals aboard the flight restrained the man and he was questioned. The plane landed safely as military jets were scrambled.
Hours after the plane landed, the man was still being interviewed by investigators, and it was unclear what if any charges he might face.
Jayne Smith, 61, said her husband, Scott, was on the United flight and was being interviewed by authorities.
She said he called her and said he didn't see anything on the flight because the incident occurred in the first-class section and he was at the back of the plane.
"He just said someone had a temper tantrum," Ms Smith said.
She said her husband told her that he and the other passengers were being interviewed at a fire station.
Emergency vehicles' flashing red lights surrounded a fire station at Denver International Airport, and two buses, apparently carrying passengers from the flight, left the station for an unknown location.
Inside the terminal, passengers from other flights picked up their luggage at baggage carriers, apparently unaware of any emergency.
The airport remained open during the incident, and spokesman Jeff Green said no flights were delayed or cancelled.
A senior State Department official said the agency was aware of the tentative identification of the man as a Qatari diplomat and that there would be "consequences, diplomatic and otherwise" if he had committed a crime.
The latest edition of department's Diplomatic List, a registry of foreign diplomats working in the United States, identifies a man named Mohammed Yaaqob Y.M. Al-Madadi as the third secretary for the Qatari Embassy in Washington.
Third secretary is a relatively low-ranking position at any diplomatic post and it was not immediately clear what his responsibilities would have been.
Foreign diplomats in the United States, like American diplomats posted abroad, have broad immunity from prosecution.
The official said if the man's identity as a Qatari diplomat was confirmed and if it was found that he may have committed a crime, US authorities would have to decide whether to ask Qatar to waive his diplomatic immunity so he could be charged and tried.
Qatar could decline, the official said, and the man would likely be expelled from the United States.
The Boeing 757 was carrying 157 passengers and six crew members, said Michael Trevino, a spokesman for United Airlines.
The flight left Reagan National Airport in late afternoon and landed in Denver International Airport in the evening, Mr Trevino said.
The flight crew radioed air traffic control to ask that the flight be met on the ground by law enforcement, he said.
A law enforcement official later said there will be no criminal charges against the man.