It was unclear what the heart was intended for and whether anyone's life was ever in danger. But the organ was supposed to be left in Seattle. 
 
 BREAKING, 12:33 p.m., Thursday: We now know where Seattle’s airborne heart was headed after the Dallas-bound flight was turned around. 
 
 On Sunday afternoon, a Southwest Airlines flight bound for Dallas made a hairpin turn over eastern Idaho and headed back for Seattle-Tacoma International Airport. The reason, the captain told passengers: Someone forgot to unload a human heart. 
 
 Dr. Andrew Gottschalk recalled that his fellow passengers went through a series of reactions to the news, the first shock. A human heart being transported on a commercial carrier? But the next reaction was of kindness because everyone on board “was happy to save a life,” he said. 
 
 The captain went on to explain that the heart should have been left in Seattle after an earlier flight from Sacramento. 
 
 Then horror sank in, Gottschalk said, as some passengers with an internet connection began to research how long a heart could be viable for a transplant — mere hours. 
 
 Southwest confirmed that the flight had to return to Seattle on Sunday afternoon after officials realized the plane was still carrying the heart intended for delivery to a hospital. 
 
 But additional details, including its intended destination and what it was being used for, remain unclear. Also unknown is whether anyone’s life was ever in danger. 
 
 Southwest flight 3606 landed at Sea-Tac after about three hours in the air, and the “life-critical cargo shipment” was unloaded from the plane, Southwest spokesman Dan Landson said by email. He added that Southwest made the decision to return because it was “absolutely necessary to deliver the shipment to its destination in the Seattle area as quickly as possible.” 
 
 “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. 
 
 The sender was a company that specializes in shipments that are “life critical,” which can mean organs for transplant, medications or specimens for treatments, said Southwest, which did not provide the name of the company. 
 
 But no Seattle-area hospitals said they were involved. Spokeswomen for regional organ-procurement organizations in Washington and California both said they never use commercial flights for heart transplants. 
 
 “We only use private flights,” said Katherine Pliska, spokeswoman for LifeCenter Northwest, the organization that facilitates the transfer of organs for transplant in the region. “There’s a time limit to get where it needs to go.” 
 
 Southwest and many other airlines work with companies that ship organs for transplants, among other perishables such as human remains, to maximize revenue, though it accounts for less than 1 percent of Southwest’s total revenue, the Dallas Business Journal reported last year. Other parts of a heart, such as heart valves and vessels, can be recovered when a whole heart transplant isn’t feasible, according to LifeCenter Northwest. 
 
 Passenger Gottschalk, a doctor who cares for professional athletes at his medical practice in New Orleans, called the incident a “horrific story of gross negligence,” no matter where the heart was intended to go. 
 
 “The heart in question traveled from California, to Washington, to the other side of Idaho, and back to Washington,” he said. 
 
 Once on the ground, passengers were also told to deplane — the aircraft had an unrelated mechanical issue, Landson said. After a five-hour delay, the passengers once again took off for Dallas. ||||| Please enable Javascript to watch this video 
 
 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. ||||| (CNN) A Southwest flight was forced to turn around after a human heart meant for donation was accidentally left on board, the airline said. 
 
 The heart had arrived Sunday at Seattle-Tacoma International Airport, where it was due to be unloaded, officials said. 
 
 But it wasn't until after the pilot took off for Dallas that the airline realized a "life-critical cargo shipment" was still on the aircraft, Southwest said in a statement. 
 
 "We made the decision to return to Seattle to ensure the shipment was delivered to its destination within the window of time allotted by our cargo customer," the airline said. 
 
 Despite the delay, the heart was unharmed and reached its destination on time, according to Sierra Donor Services, the nonprofit that organized the donation. 
 
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– The passengers on Southwest Flight 3606 were somewhere over Idaho Sunday when their captain came over the PA with a message: They were headed back to Seattle. That's because there was a "life-critical cargo shipment" that had been left on board, per the Seattle Times: a donated human heart. The heart should have been left in Washington after arriving from Sacramento, but instead it stayed on the plane and was Dallas-bound before the airline realized it hadn't been removed at Sea-Tac Airport. CNN and its affiliate KTXL note that this heart wasn't intended for a full transplant; it was donated for its valves alone. That means a) it was in a thermally controlled box in the cargo hold, making it easier to forget, and b) it had, per KTXL, a "much longer transportation window" than if it had been intended for a heart transplant. Luckily, no damage came to it, and the heart got to where it needed to go on time, per the Sierra Donor Services nonprofit. The passengers, meanwhile, weren't too irked, as they were "happy to save a life," one man on board tells the Times. (Another organ donor was 107 years old.)