But the Seattle Times reported that the flight's captain told passengers the cargo was a human heart. Instead, the plane was forced to turn around because a human heart was left on the plane.
The airliner was flying over eastern Idaho - around 600 miles (950 kilometers) into the journey - when staff discovered the "life-critical cargo shipment", which had been transported from California and was meant to be delivered to a Seattle hospital. But no Seattle-area hospitals said they were involved.
"We only use private flights", Katherine Pliska, spokeswoman for LifeCenter Northwest, the organization that facilitates the transfer of organs for transplant in the Pacific Northwest, told The Seattle Times.
Once on ground, the flight was taken out of service due to an unrelated mechanical issue.
While the heart should have been taken out of the cargo hold in Seattle, it was mistakenly left on the Southwest Airlines connecting flight, USA Today reported.
A doctor on board the flight - who had nothing to do with the heart - said the incident was a "horrific story of gross negligence".
A Southwest Airlines pilot pulled a U-turn to make an unplanned medical delivery to Seattle. Deanna Santa of Sierra Donor Services in Sacramento, California, confirmed to the outlet that the heart was being sent to the tissue processor to recover a valve, which would be used in a future transplant, but did not yet have a designated recipient. But the shock gave way to kindness, and many onboard were "happy to save a life".
The heart, which had been on an earlier flight from Sacramento to Seattle, was not removed from the plane as it should have been.
Commercial airlines are known to work with companies that ship organs intended for transplant.