The yellow Labrador retriever appeared woebegone, his head slack over his front left paw, his muzzle resting on the ground. Yet he was also steadfast, still keeping watch over George H.W. Bush, the 41st president, who died on Friday at his home in Houston.

After accompanying the statesman and World War II veteran in the final months of his life, Sully, the late president's service dog, lay before the casket holding what remained of him.

The display of instinctual, animalistic devotion captured the reaction to Bush's death in a way that the words spilled all weekend over the Internet could not. Dogs, wrote the poet Emily Dickinson, "know but do not tell."

In his knowing pose, the dog was at rest. He will accompany his person a final time, as Bush's body is flown from Houston to Washington, CNN reported. An arrival ceremony is expected Monday at the Capitol, where Bush will lie in state in the rotunda until Wednesday, when family and friends will gather for a funeral at Washington National Cathedral.

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"Mission complete," Jim McGrath, the Bush spokesman who shared a photo of the service dog's mournful stance on Sunday, wrote on Twitter.

The moving image also appeared on the Facebook page of America's VetDogs, a service dog program that assists veterans, active-duty service members and first responders struggling with disabilities. The Labrador was raised by VetDogs, first through its prison puppy program that gives inmates the chance to teach animals the basic tasks of housebreaking and standardized commands, and then at the program's campus in Smithtown, New York.

Sully was matched to Bush in June of this year, at the age of 2. He is named after former airline pilot Chesley "Sully" Sullenberger, who safely landed a plane on the Hudson River in 2009.

Bush, a Republican, welcomed "the newest member of our family" the same day he received a visit from a friend and onetime political adversary, Bill Clinton, at his home in Kennebunkport, Maine. The occasion was also distinguished by a sartorial salute made by Bush to his successor. He wore a pair of socks imprinted with Clinton's face.

A form of Parkinson's disease confined the former president to a wheelchair or motorized scooter in the final years of his life. Among the services that Sully was able to perform for Bush were retrieving dropped items, opening and closing doors, pushing an emergency button and supporting him when standing. As the animal went about these tasks, he amassed a following on social media, including on his own Instagram account, which boasts more than 79,000 followers as of early Monday.

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Sully celebrated his birthday in July with a bone tied up in a bright pink bow. Last week, he was already preparing for Christmas. On Sunday, the photo of the service dog lying before Bush's casket became the latest post.

While his presidential mission is complete, Sully's work isn't done. America's VetDogs said the Labrador would join the Walter Reed General Hospital in Bethesda, Maryland, where he would assist with "physical and occupational therapy to wounded soldiers and active duty personnel during their journey to recovery."

That Sully would continue to serve veterans was a source of comfort for the Bush family. The late president's son and the 43rd president, George W. Bush, predicted that the dog would bring "joy" to patients at Walter Reed. Jeb Bush, the president's younger son and a 2016 contender for the Republican presidential nomination, also weighed in.

"Sully has the watch," he wrote on Twitter.

The 41st president's devotion to dogs was not born of necessity alone. Long before he came to rely on man's best friend to move around, he and Barbara Bush, who died in April, kept the company of Millie, an English springer spaniel named for Mildred Caldwell Kerr, a friend of the couple. The pet was once declared "the most famous dog in White House history."

Bush did, however, use the dog to score political points, though these didn't always land effortlessly.

In a 1992 campaign speech, the incumbent Republican president attacked Democrats Bill Clinton and Al Gore by saying, "My dog Millie knows more about foreign affairs than these two bozos."

Sully, by contrast, bore none of the weight of his person's political ambitions. Sometimes he appeared with the American flag, such as on the Fourth of July. At other points, he was used to promote guide dogs.

But mostly he was just at Bush's side, which is where he remained after the president's death.