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.
The incident comes 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.
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 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 sources asked not to be identified because they were not authorized to discuss the ongoing investigation.
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. Officials have not declared an "all clear," but indications are this was not a terror-related incident, reports CBS News correspondent Bob Orr.
Another passenger on the United flight, 61-year-old Scott Smith of Laramie, Wyo., said a man was led off the plane in handcuffs from the first-class area after the plane taxied to a stop.
Passengers remained on the plane for nearly an hour before they were taken off for questioning, and they were told to leave their carryon items on the aircraft, he said.
Smith, a computer programmer, said he was seated toward the middle of the plane and didn't notice any disturbance during the flight.
"The approach into Denver was unusual. We came in rather fast, and we were flying low for a long period of time. I've never seen a jetliner do that," Smith told reporters by cell phone. "There were no announcements, nothing about your carryon bags or tray tables."
Once on the ground, Smith said, the pilot eventually announced that "we have a situation here on the plane."
"We sat there for a long time, almost an hour," Smith said.
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.
The flight crew radioed air traffic control to ask that the flight be met on the ground by law enforcement, Trevino said.
The airport remained open during the incident, and no flights were delayed or canceled, airport spokesman Jeff Green said. Inside the terminal, passengers from other flights picked up their luggage at baggage carriers, apparently unaware of any emergency.
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, U.S. 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.
Qatar, about the size of Connecticut and Rhode Island combined, is an oil- and gas-rich monarchy and close U.S. ally of about 1.4 million people on the Arabian peninsula, surrounded by three sides by the Persian Gulf and to the south by Saudi Arabia.
The country hosts the forward headquarters of the U.S. Central Command, which runs the wars in both Afghanistan and Iraq, and is major supporter of operations deemed critical to both campaigns. It also played a prime role in the 1991 Gulf War, which drove Saddam Hussein's Iraq out of Kuwait.