The mock improvised explosive device was discovered in a man's suitcase at the Toronto Preclearance facility, a spokesperson for the U.S. Customs and Border Protection said.
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"They were saying there's a security breach, but we're sitting on the plane for three hours, so we're just kind of like, what's going on?" said passenger Zachary Brincheski.
"I texted my husband and said I'm really scared now because they're taking us away from the airport, and I'm worried that there might be a bomb on the plane," said Kimberley Thomas, a passenger.
CBP officers notified the Canadian Air Transport Security Authority, who swabbed the mock IED for explosives with a negative result.
Screening was temporarily halted at the preclearance facility while the mock IED was tested, the CBP spokesperson said.
The passengers on United Airlines Flight 547 were re-screened and normal operations have resumed at the airport.
The suspect has been identified as Joseph Galaska, according to police in the Peel region of Ontario.
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The device was not part of a training exercise.
The actual plane flew on to O'Hare without any passengers and just the crew so that the plane could be used for its next scheduled flight.
Passengers boarded onto United Airlines 2709 and provided with snacks and drinks before departing. United will be issuing compensation for the disruption.
United Flight 2709 arrived at O'Hare in the afternoon.