An economics professor at the University of Pennsylvania was profiled and interrogated this week while sitting on the tarmac on an American Airlines flight to Syracuse...all because his seatmate became concerned about the "foreign lettering" he'd been scribbling on paper next to her. That foreign lettering, it turns out, was math.

As first reported by the Washington Post, Guido Menzio, a celebrated economist who's taught at Princeton and Stanford in addition to the University of Pennsylvania, was questioned by American Airlines interrogators on Thursday, after he was removed from American Eagle Flight 3950 in Philadelphia. Menzio, who is from Italy, was informed he'd been suspected of terrorism, after his 30-something year old female seatmate became concerned about the unrecognizable "code" he'd been writing on a notepad.

The woman apparently claimed she was ill, flagged down a stewardess, and reported Menzio. The plane, which had been prepared to take off, returned to the gate at the airport—Menzio's seatmate, who has not been identified, got off the flight, and Menzio was taken off the plane and “met by some FBI looking man-in-black”.

"They ask me about my neighbor," he wrote in a post on Facebook. "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 was working on equations related to a paper on menu costs and price dispersion that he planned to present at Queen's University in Ontario, where he was headed via connecting flight in Syracuse. When he showed interrogators he was writing math equations and not, say, Arabic or code, he was allowed to return to his seat, and the Washington Post reports that "the pilot seemed embarrassed." Menzio's seatmate did not re-board the flight, which was delayed by two hours thanks to the incident.

Menzio told reporters he was "treated respectfully throughout,” but he wasn't thrilled with the xenophobia that led to his interrogation. "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," he said. "What might prevent an epidemic of paranoia? It is hard not to recognize in this incident, the ethos of [Donald] Trump’s voting base."