The passenger found the shell at 6:47 a.m. local time in the seat-back pocket in front of him, said Fran McLaughlin, a spokeswoman for the Milwaukee County Sheriff's Office. The incident occurred at Milwaukee's General Mitchell International Airport.

A sheriff's canine unit swept the plane and found no other shell casings, McLaughlin said.

The Transportation Security Administration rescreened all the passengers aboard the flight. After a one-hour delay, the plane took off for its original destination of Detroit, she said.

Representatives from Delta and the TSA could not be reached for comment.

The man who found the shell was interviewed by investigators, who found nothing suspicious about him, McLaughlin said. It remained unclear how the shell ended up on the flight. (Reporting by Brendan O'Brien in Milwaukee and Alex Dobuzinskis in Los Angeles; Editing by Tim Gaynor and Peter Cooney)

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