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SACRAMENTO — Southwest Airlines says a Dallas-bound flight returned to Seattle last weekend because a human heart was left onboard.

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

The heart, gifted to someone in need of its valves, left West Sacramento’s Sierra Donor Services over the weekend headed to Seattle. That’s where LifeNet Health Inc. was set to remove the valves for a future surgery.

It’s a process the company does smoothly with many different kinds of tissue 1,000 times a year but this time was different. The courier LifeNet Health contracted got the box on board its designated Southwest flight.

“My understanding is the pilot made an announcement when they turned around that they were returning because there was a heart that had been left on board that they needed to get back to the hospital,” said Sierra Donor Services Public Education and Relations Manager Deanna Santana.

That set-off a social media firestorm as doctors and fellow passengers who happened to be on board started raising concerns online and otherwise that the lost heart wouldn’t be viable.

“They emailed some reporters and the story ensued,” Santana said.

But it was a story with a lot of misinformation.

“No one on either the tissue processing side or on the organ donation side or on the tissue donation side missed a beat,” Santana explained. “We did everything we possibly could to make sure those gifts got to the right place and in the end, they ultimately did get to the right place.”

Tissue slated for transplant as opposed to a whole organ has a much longer transportation window. Donated heart valves once processed can wait up to four years before they’re implanted.

Donated tissue also doesn’t travel with a person inside a plane’s cabin. It flies in a thermally-controlled box in the cargo hold.

While a courier for LifeNet waited for pickup in Seattle, the tissue was left behind, just like a suitcase.

In her 30 years in the field, the director of Sierra Donor Services said she’s never seen anything like this happen. Similarly, Santana told FOX40 she has not seen it anywhere in her seven years.

However, Santana has seen the power of tissue and organ donation with her own son.

“He saved five lives,” Santana said. “He gave sight to two people and he helped 73 people with his tendons and bones.”

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

The Associated Press contributed to this report.