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. A law enforcement official said there will be no criminal charges against the man.
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 later, the man was still being interviewed by investigators.
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.
Passengers say they were kept on the plane for nearly an hour after it landed and then questioned at a fire station at the airport.
Passenger Mei Turcotte, 26, of Kalispell, Mont., told The Associated Press she smelled smoke about an hour into the flight. She said she later looked out the window and saw two jets flying alongside the plane.
"I'm in the sky a lot, and I was thinking that might not be so normal," she said.
She was angry about being held against her will to be questioned over something so minor.
"He went quietly. There was not a scene," Turcotte said. "They made this into something that was ridiculous."
Dave Klaversma, 55, of Parker, Colo., said his wife, Laura, was sitting behind the man in the first-class section of the United flight. She said she saw the man go into the bathroom and that moments later he said something to the flight crew. After that, two U.S. marshals in the first-class section apprehended the man and sat next to him for the remainder of the flight.
Klaversma said his wife told him it all happened very quietly and that "there was no hysteria, no struggle, nothing." She said she noticed nothing unusual about the man before the incident.
Another passenger, 61-year-old Scott Smith of Laramie, Wyo., 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," Smith, a computer programmer, told reporters by cell phone. "We came in rather fast, and we were flying low for a long period of time. I've never seen a jetliner do that. 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."
The Transportation Security Administration confirmed that federal air marshals responded to a passenger "causing a disturbance onboard the aircraft," but didn't elaborate.
"Law enforcement and TSA responded to the scene and the passenger is currently being interviewed by law enforcement," TSA said in a statement. "All steps are being taken to ensure the safety of the traveling public."
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 went through security and picked up their luggage at baggage carriers, apparently unaware of any emergency.
Erin Montroy, who was passing through the airport on her way from Kansas City to Las Vegas, said she hadn't heard anything about the incident and wasn't alarmed.
"I don't really ever feel as threatened as they think we should," she said.
President Barack Obama was briefed about the incident aboard Air Force One by National Security Adviser Gen. Jim Jones and National Security Chief of Staff Denis McDonough shortly before 9 p.m. EDT, said a White House official, who spoke on the condition of anonymity to discuss an ongoing investigation. The president is traveling to Prague, where he'll sign a nuclear arms treaty with Russia Thursday.
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.
An online biography on the business networking site LinkedIn shows that a Mohammed Al-Madadi has been in Washington since at least 2007, when he began studying at George Washington University's business school.
The job title listed on the site is database administrator at Qatar's Ministry of Foreign Affairs.
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.
Associated Press writers Eileen Sullivan, Matthew Lee, Matt Apuzzo, Joan Lowy, Pauline Jelinek and Lolita C. Baldor in Washington and Judith Kohler, Thomas Peipert and David Zalubowski in Denver contributed to this report.