(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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