A Southwest Airlines flight headed from Seattle to Dallas was turned back mid-flight after it was discovered that a human heart had been left on board, officials said Thursday.

The Seattle Times reported that Flight 3606 arrived in Seattle from Sacramento, California. Someone forgot to unload the heart before the plane left for Dallas, and the captain announced over Idaho they were turning back.

The heart was being sent to a tissue processor to recover a valve for use in a future transplant.

Deanna Santa of Sierra Donor Services in Sacramento, California, said the organ-procurement organization sent the heart through a courier, who picked it up in Sacramento for shipment to Seattle. The delay did not impair the heart's usefulness, and no patient needed it imminently.

Andrew Gottschalk, a doctor with no connection to the incident, described fellow passengers' shock as those with internet connections discovered the small window of viability for a heart transplant.

He told the Times the incident was a "horrific story of gross negligence."

The flight took off again for Dallas after a five-hour delay.