When the marshals demanded to know what he was doing, the man allegedly said "I'm lighting my shoes on fire." The two marshals then went subdued the man, identified as 27-year-old Mohammed Al-Madadi, a diplomat in the Qatar embassy in Washington.
"There was no Steve Siegel B.S. I mean, these guys talked to him. He sat down, he was completely compliant with them," said one passenger.
It turned out Al-Modadi was trying to sneak a cigarette in the plane's lavatory, and his remark was meant as some sort of joke.
Al-Modadi's comment set off a chain reaction of security and terrorism responses, including F-16 fighter jets that scrambled from Peterson Air Force Base in Colorado to accompany the flight during its descent into Denver. The plane was met by the FBI and local law enforcement.
The 757 jetliner was held in Denver for several hours as FBI agents interviewed Al-Modadi and the other passengers. Al-Modadi's shoes were examined and no explosives were found on the flight.
"He called and said 'there's been an incident, some guy got mad in first class and had a tantrum.' That's the way he described it, he was kind of chuckling about it. He's just more annoyed that he had to sit out there for an hour and a half so it didn't sound like it was very alarming," said Jane Smith, whose husband was on the flight.
Federal officials said Mohammed Al-Modadi was on official business when he boarded the United Airlines flight, giving him diplomatic immunity. Authorities said they don't think the envoy was trying to hurt anyone. He has been released and was headed back to Washington.
A statement was posted on the Qatar Embassy's Web site in response to the incident, defending Al-Modadi and calling it a mistake.
"We respect the necessity of special security precautions involving air travel, but this diplomat was traveling to Denver on official embassy business on my instructions," Ambassador Ali Bin Fahad Al-Hajri wrote in the statement, "and he was certainly not engaged in any threatening activity."
"The facts will reveal that this was a mistake," the statement continued, "and we urge all concerned parties to avoid reckless judgments or speculation."
The Boeing 757 was carrying 157 passengers and six crew members, United Airlines spokesman Michael Trevino said. It left Reagan National Airport at 5:19 p.m. EDT and landed at Denver International Airport at 7 p.m. MDT.
Security was temporarily tightened at Los Angeles International Airport when news of the incident came in. Extra patrols were added to the terminal buildings and out in the field around the perimeter of the airport. But those security measures were lifted once it became clear that this was an air scare involving a smoker and not a terrorist.
Wednesday's scare came three months after the attempted terror attack on Christmas Day when a Nigerian man tried to blow up a Detroit-bound airliner. Since then, law enforcement, flight crews and passengers have been on high alert for suspicious activity on airplanes. The scare exposed major holes in the country's national security and prompted immediate changes in terror-screening policies.
Two law enforcement officials said investigators were told the man was asked about the smell of smoke in the bathroom 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.
The number of in-flight air scares has prompted airline pilots to divert their flights twice as much this year as last year. In the first three months of 2010, 35 flights in the U.S were diverted to other airports because of some sort of incident abroad the plane, as compared to 17 in the same period last year.
The Associated Press contributed to this report.