A Southwest Airlines flight bound for Dallas turned around and headed back to Seattle when crew members realized an important piece of cargo hadn't been unloaded: a human heart.

The mistake was discovered Sunday somewhere over Idaho, and the plane executed a sharp turn and returned to Seattle-Tacoma International Airport, the Seattle Times reported.

The heart was being sent to an area tissue processor to recover a valve.

Dan Landson, a spokesman for the Dallas-based airline, said the flight was in the air for about three hours before it returned to Seattle. He said the carrier made the decision to head back because it was "absolutely necessary to deliver the shipment to its destination in the Seattle area as quickly as possible."

Deanna Santa of Sierra Donor Services in Sacramento, Calif., told the Times that the organ-procurement organization sent the heart through a courier, who picked it up in Sacramento for shipment to Seattle. The valve will be used for a transplant, though there is no designated patient yet.

"The most important part is that no one was waiting," she told the newspaper, adding that once it was back in Seattle the heart was taken to for tissue processing. It wasn't affected by the delay, she added.

"Nothing is more important to us than the safety of our customers and the safe delivery of the precious cargo we transport every day," Landson said in an email to the Seattle Times.

The turnaround created a five-hour delay for travelers, according to the newspaper.

A doctor on board the flight -- who had nothing to do with the heart -- said the incident was a "horrific story of gross negligence."

"The heart in question traveled from California, to Washington, to the other side of Idaho, and back to Washington," Dr. Andrew Gottschalk told the newspaper.

No one complained about being turned around, Gottschalk said, but passengers with internet connections quickly became alarmed when they found out a human heart is usually viable for a transplant for only a few hours, the newspaper reported.

It is unclear whom the heart was intended for. Seattle-area hospitals that were contacted said they knew nothing about a possible heart transplant, the newspaper reported.