An Italian economist says his flight was delayed after a fellow passenger saw him working on a differential equation and alerted the cabin crew.
Guido Menzio was taken off and questioned by agents who did not identify themselves, after the woman next to him said she felt ill.
He showed them what he had been writing and the flight eventually took off – more than two hours late.
Mr Menzio told the Washington Post that the pilot seemed embarrassed.
He wrote on Facebook that the experience was “unbelievable” and made him laugh.
The University of Philadelphia associate professor boarded the Philadelphia-Syracuse flight on Thursday on his way to Ontario, where he was due to give a lecture.
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.
He wrote: “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.
“Because of that an entire flight was delayed.”
He told Associated Press that the crew should have run additional checks before delaying take-off.
He said: “Not seeking additional information after reports of ‘suspicious activity’ is going to create a lot of problems, especially as xenophobic attitudes may be emerging.”
American Airlines, whose regional partner Air Wisconsin was operating the flight in question, said the crew followed protocol to take care of an ill passenger and then to investigate her allegations. It was established that they were not credible.
The woman was re-booked on a later flight.