Mohammed al-Madadi boarded a flight from Washington to Denver on Wednesday afternoon, a junior diplomat from Qatar on a routine assignment to visit a Qatari citizen in a U.S. prison. United Airlines Flight 663 passed uneventfully until Madadi stepped out of the bathroom.

A flight attendant smelled smoke and confronted Madadi, 27, who joked that he "was trying to light my shoes on fire" to mask the smell of the bathroom before proceeding to his seat, law enforcement officials said. The attendant challenged Madadi again, one official said, and when he declined a request to hand over his lighter, the attendant notified the air marshals on the plane.

That is when Madadi's business trip to Denver started to become a minor international incident.

The marshals talked to Madadi briefly, confined him to his seat, and activated a national alert system through the pilot. Fighter jets were scrambled, and President Barack Obama was warned about a possible terrorist threat.

After questioning Madadi on the ground and finding no explosives, authorities said there had been no offense beyond illegal smoking, a charge from which he is immune because of his diplomatic status. Officials said he had been smoking a small pipe in the plane's bathroom.

As Madadi was returning to Washington on Thursday, accompanied by embassy staff members, the State Department made it clear to the Qatari government that it would declare him persona non grata and expel him if Qatar did not remove him. U.S. officials said Qatar, one of the closest U.S. allies in the Persian Gulf region, will send him home.

The scare came as the United States has increased aviation security since a suspected al-Qaida operative from Nigeria allegedly tried to blow up a Detroit-bound jetliner on Christmas Day using explosives hidden in his underwear. Since December, aviation terrorist alert levels have been raised and marshals have been added to more flights. The Transportation Security Administration said that 33 flights were diverted for security reasons in the first three months of this year, a pace that is running 20 percent higher than each of the past five years.

In December 2001, Richard Reid, a British citizen who trained at an al-Qaida camp in Afghanistan, attempted to detonate a homemade bomb hidden in his shoes on a flight from Paris to Miami. He pleaded guilty and is serving a sentence of life in prison without parole.

On Thursday, Homeland Security Secretary Janet Napolitano commended the unnamed marshals aboard the flight to Denver and said they "took appropriate and immediate action to secure the aircraft and communicate the potential threat to authorities on the ground."

But it was unclear whether the incident would have escalated to a full-blown terrorist alert if the marshals, who travel only on certain flights, had not been aboard. Smoking is a common violation aboard aircraft worldwide, as are disruptive passengers, but industry statistics indicate that very few episodes lead to law enforcement involvement.

According to a database of 10,000 unruly-passenger reports compiled by the International Air Travel Association, collected from more than 100 airlines worldwide since 2007, unauthorized use of cigarettes or illegal drugs makes up about a third of the cases. Of them, 17 percent led to intervention by security staff or police, and 24 cases went to court. Only one of those cases led to a fine and a conditional sentence.

As word of Madadi's fate traveled quickly through Washington's diplomatic community, the reaction at Arab and Muslim embassies was twofold. There was widespread agreement that Madadi appeared to have done a dumb thing. But many think that profiling was involved, and that the situation would not have gone so far if Madadi were not Arab.

"We all share the same concerns about profiling of Arabs and Arab Americans in a time of strong anxiety, particularly in the air travel industry," said one ambassador. Enhanced security, he said, is in "everyone's best interest, and we hope and expect the security procedures are implemented uniformly."

One of his colleagues was more blunt, saying that the incident "never would have happened if (Madadi) were Swedish."

Several diplomats from Arab and Muslim-majority countries said they had been stopped while boarding domestic flights in the United States, many for secondary searches and pat-downs they think were initiated because of their names or nationalities. "They're not supposed to do pat-downs of ambassadors," said one chief of mission, "but my choice is to argue about it or just get it over with."

"I have a U.S. passport" in addition to that of the country he represents, another diplomat said. "But I've had situations where I'm traveling with a blond, blue-eyed person and I'm the one who gets pulled out for secondary checking."

The administration announced last week that it would drop a requirement, instituted after the Christmas Day incident, of physical searches and additional baggage checks for U.S.-bound international passengers from or traveling through 14 predominantly Muslim nations. The move was in response to protests from governments, U.S. Muslim organizations and civil liberties groups.

Madadi, the son of a Qatari diplomat, attended high school in Virginia and graduated from Marymount University. He received a master's degree in information systems from George Washington University in 2008, after which he returned to Qatar and joined its foreign service. He arrived here late last summer, said a friend of his who spoke on the condition of anonymity. "He finished his apartment and got a baby bulldog," the friend said. "He just got his new car last week."

Madadi, who served at the embassy for less than a year in what was his first overseas assignment, was on his way to make a consular visit to Ali al-Marri, a Qatari national serving an eight-year sentence for terrorism offenses at the "supermax" prison in Florence, Colo.

Staff writer Laura Blumenfeld and staff researcher Julie Tate contributed to this report.