A human heart was left onboard Southwest (LUV) flight 3606 heading to Dallas from Seattle on Sunday afternoon, the Seattle Times reported. According to the outlet, the flight was forced to turnaround midflight when officials realized the heart was onboard the plane, much to the shock of passengers.

The heart was intended for arrival at Seattle hospital from a flight that previously came from Sacramento. Passengers onboard the flight were quick to use their cell phones to learn how much time the human heart had to remain viable for the person receiving it, the Seattle Times learned. A doctor aboard the flight, Andrew Gottschalk, told the paper that he thought the entire incident was a “horrific story of gross negligence.”

The paper reported that that the heart had made its way from California to Washington, across Idaho and back to Washington. The flight landed back at the Sea-Tac Airport after spending approximately three hours in flight.

Dan Landson, a Southwest spokesperson, said it was “absolutely necessary to deliver the shipment to its destination in the Seattle area as quickly as possible.” He went on to say, “Nothing is more important to us than the safety of our customers and the safe delivery of the precious cargo we transport every day.”

Information on the name of the company that shipped the heart was not provided by Southwest, but the company did confirm that the airline does specialize in shipments like these that are “life critical,” according to the Seattle Times. No details on whether the heart would be used to save a life were provided.

The human heart was unloaded from the plane in Seattle, where passengers were then asked to deplane the aircraft as it was having an unrelated mechanical issue, Landon told the outlet. Passengers were able to depart for Dallas after a five-hour delay.

As of this afternoon, Southwest stock was down about 2 percent.