WASHINGTON (AP) — The day that panda lovers have been dreading has finally come.

The National Zoo’s three giant pandas, Mei Xiang, Tian Tian and their cub Xiao Qi Ji. on Wednesday began their long trip to China, leaving behind an empty panda exhibit with no certainty that pandas ever would again take up residence there.

“It is a moment with some heartbreak in it,” said National Zoo Director Brandie Smith. “But it is also a moment of joy because we are celebrating the success of the world’s longest running conservation program for a single species.”

The pandas had been loaded into large white crates away from the public view. Forklifts took them to waiting trucks — along with several bushels of bamboo for road snacks. Their faces were occasionally visible through a small window at the side of the crates. Zoo personnel walked alongside them.

They were taken to Dulles International Airport in northern Virginia, where a specially outfitted Boeing 777F dubbed the FedEx Panda Express was waiting. The 19-hour flight to Chengdu, China will include a refueling stop in Anchorage, Alaska.

Smith noted that during the 50 years of panda exchanges between China and the National Zoo, giant pandas have been removed from the list of endangered species.

She said she hopes to be back at the airport “sometime soon again as we celebrate the return of giant pandas to Smithsonian’s National Zoo.”

But there’s no guarantee that pandas will return to the zoo. The zoo’s exchange agreement with the China Wildlife Conservation Association expires in early December and negotiations to renew or extend the deal have not produced results.

Veteran China-watchers have speculated that Beijing is gradually withdrawing its giant pandas from zoos in America and Europe amid diplomatic tensions with a number of Western governments.

The San Diego Zoo returned its pandas in 2019, and the last bear at the Memphis, Tennessee, zoo went home earlier this year. The departure of the National Zoo’s bears means that the only giant pandas left in America are at the Atlanta Zoo — and that loan agreement expires late next year.

Chinese Embassy representative Xu Xueyuan praised the benefits of the China’s multiple panda exchange agreements with zoos around the world. But she offered no hints on whether the program with American zoos will continue in the short term.

“Such collaboration has contributed strongly to the mutual understanding and friendship between the Chinese and American peoples,” she said. “China will continue to work closely with cooperation partners, including the United States, on the conservation and research of endangered species and biodiversity protection.”

The zoo’s incredibly popular livestream panda-cam is now showing greatest hits panda videos.