The plane, ready to take off, then returned to the gate and the woman passenger left.

Highlights The economist was flying to Philadelphia to Syracuse for a talk in Canada

Co-passenger passed a note to crew that she was suspicious of him

Economist made to disembark plane, questioned, later allowed on flight

An Italian economist says his flight was delayed for over two hours after a fellow woman passenger saw him working on an equation she thought was some special secret terrorist code and alerted the cabin crew.Guido Menzio, 40, an associate professor at the University of Pennsylvania, was taken off and questioned by agents in Philadelphia, after the woman next to him said she felt ill.The Ivy League economist was flying from Philadelphia to Syracuse on Thursday to give a talk at Queen's University in Ontario, Canada.American Airlines confirmed yesterday that a 30-something woman expressed suspicions about Mr Menzio. She said she was too ill to take the Air Wisconsin-operated flight 3950.Before the flight took off, the woman sitting next to him passed a note to a member of the cabin crew.She initially told them she was feeling unwell but then voiced her suspicions about Mr Menzio's scribblings.The plane, ready to take off, then returned to the gate and the woman passenger left.Mr Menzio was then asked to disembark the plane and "met by some FBI looking man-in-black".He was solving a differential equation, but said he was told the woman thought he might be a terrorist because of what he was writing.American spokesman Casey Norton said the crew followed protocol to take care of an ill passenger and then to investigate her allegations. They determined them to be non- credible, he said.Mr Menzio showed the security agents what he had been writing and the flight eventually took off - more than two hours late.He wrote on Facebook that the experience was "unbelievable" and made him laugh."It's a bit funny. It's a bit worrisome. The lady just looked at me, looked at my writing of mysterious formulae, and concluded I was up to no good," he wrote.He told the Washington Post that he was "treated respectfully throughout" the process but remains perturbed by a system that "relies on the input of people who may be completely clueless".The woman was re-booked on a later flight.