An olive-complexioned, curly-haired University of Pennsylvania economics professor was deeply focused while scribbling an algebraic equation Thursday night, waiting aboard an American Airlines flight scheduled to take off from Philadelphia to Syracuse, N.Y.

He didn't have time to talk to the passenger next to him - a blond-haired woman wearing flip-flops who appeared to be in her 30s.

His behavior, his looks, and the little that he said to his seatmate apparently frightened her. She passed a note to a flight attendant.

The next thing that Guido Menzio, 40, knew, the plane had returned to a gate at Philadelphia International Airport. Menzio - who is Italian - was met by what he described as an "FBI looking man-in-black."

He was told the passenger thought he was a terrorist.

Menzio, who did not return an email seeking comment Saturday, wrote about his experience in a social media post, and his story was chronicled in a Washington Post article.

After authorities realized the woman's concerns about Menzio were not credible, Flight 3950 took off - minus the complaining passenger and after a more-than-two-hour delay.

Menzio, a prominent economist, has been a member of Penn's faculty since 2005, after getting his Ph.D. from Northwestern University.

Last year, he won the Carlo Alberto Medal for Best Italian Economist Under 40.

In a Facebook post that was later deleted, Menzio wrote that the woman had passed the note to a flight attendant, and that when the attendant returned, she asked the woman if she was comfortable taking off or was "too sick."

Menzio noted that the plane then returned to the gate and the woman left her seat.

He was then asked by the pilot to get off the plane, and when he did, he wrote that he was met by someone who looked like an FBI agent.

After first being asked about the woman who had been sitting next to him, Menzio said, he was then told that the woman "thought I was a terrorist because I was writing strange things on a pad of paper. I laugh. I bring them back to the plane. I showed them my math."

He told the Post in an article published Saturday that he is 40, that he was wearing jeans and a red Lacoste sweater, and that the woman had tried to make small talk with him, but he was too busy with his math notations.

Menzio, who lives in Philadelphia, was on his way to Syracuse for a connecting flight to Ontario to give a talk Friday at Queen's University in Kingston at its 2016 QED Frontiers of Macroeconomics Workshop. His 11 a.m. talk was entitled "The (Q, S, s) Pricing Rule."

Apparently, his math is pretty high-level and technical.

In his own social media post, Menzio wrote: "The lady just looked at me, looked at my writing of mysterious formulae, and concluded I was up to no good. Because of that an entire flight was delayed. . . . Trump's America is already here. It's not yet in power though. Personally, I will fight back."

The woman could not be reached. Menzio told the Post he did not know her name. The airline said it does not give out passenger information for privacy reasons.

Menzio told the Post he was "treated respectfully throughout," but was troubled by "a security protocol that is too rigid - in the sense that once the whistle is blown everything stops without checks - and relies on the input of people who may be completely clueless."

Menzio's Facebook page says he is from Turin, Italy.

Casey Norton, an American Airlines spokesman, confirmed Saturday that Flight 3950, an American Eagle flight operated by Air Wisconsin, was delayed for more than two hours Thursday night. It was scheduled to depart at 7:20 p.m.

"Taxiing out on takeoff, a customer reported she was not feeling well," Norton said. The woman asked a flight attendant for the plane to return to the gate so she could get off, he said.

He said the plane returned to the gate about 8:30 p.m. and stayed there for about an hour.

As the woman was getting off the plane, she then "reported concern about another customer's behavior," Norton said.

Norton would not say what the concern was about the other passenger. He would only confirm that the other passenger was a man and that he had been sitting near the woman.

He said American's customer-service manager, the Air Wisconsin captain, and ground security all determined the passenger's "concerns were not validated," and so the flight left the gate at 9:42 p.m.

He said the woman was rebooked on another flight to Syracuse that night.

shawj@phillynews.com215-854-2592 @julieshawphilly