A flight was delayed for two hours after a female passenger saw a mathematician working on a differential equation and alerted staff.
Guido Menzio was removed from the American Airlines flight from Philadelphia to Syracuse and questioned.
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After showing agents what he had been writing the flight took off with a two-hour delay.
Speaking to the Washington Post, he said the pilot seemed embarrassed.
Menzio was on a connecting flight to Ontario to give a talk at Queen's University.
Writing on Facebook, he said he was met by an "FBI-looking man-in-black": "They ask me about my neighbour. I tell them I noticed nothing strange. They tell me she 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."
Menzio, who is Italian, said he initially "thought they were trying to get clues about her illness."
The woman, who had passed a note to a flight attendant, left the aircraft as she was too ill to travel.
Norton added that security personnel interviewed Menzio and determined him not to be a "credible threat."