DALLAS (CBSDFW.COM) – A Southwest Airlines flight with service scheduled from Seattle to Dallas had to turn around after the crew discovered a human heart had been left onboard.

A Southwest spokesperson said the decision was made that Flight #3606 would return to Washington after “we learned of a life-critical cargo shipment onboard the aircraft that was intended to stay in Seattle for delivery to a local hospital.”

The airline didn’t say how long the plane had been in the air when the organ was discovered. The aircraft turned around to ensure the heart would make it back to Seattle within a certain time window.

It’s believed the heart made it back within the allotted time. Once the aircraft landed it was taken out of service, but for an unrelated mechanical issue.

A different plane was brought in to carry passengers on to Dallas. Passengers were delayed about five hours.