The silver pocket watch was a prized possession of Theodore Roosevelt, a keepsake given to him by his sister and her husband in 1898 before he became president that would travel with him around the world and end up at Sagamore Hill — his home on Long Island, New York, and now a national historic site.

But in 1987, it went from a museum piece to a pilfered prize when someone stole it from an unlocked case at the Theodore Roosevelt Inaugural National Historic Site in Buffalo, New York, where it was on loan.

It was a mystery that endured 36 years until it turned up at a Florida auction house last year and was seized by federal agents. On Thursday, it was returned to public display at Sagamore Hill as the National Park Service and the FBI triumphantly announced it was back home during a ceremony featuring Roosevelt’s great-grandson, Tweed Roosevelt.

“This was feel-good news,” Tweed Roosevelt, 82, said Friday in a phone interview. “For me, it kind of felt like almost as if a piece of TR’s spirit being returned to Sagamore Hill, like a little bit of him was coming back. And so I felt that was really cool.”

Growing up, he said he didn’t know about the watch and only learned about it vaguely after it was stolen. He called it “unremarkable” in appearance, but priceless to his great grandfather.

“As it turns out, this isn’t just any old pocket watch,” he said. “It was a watch that TR placed great sentimental value on.”

The mystery of the watch’s disappearance, however, is not fully solved. It is still not clear who stole it and how. The Park Service and FBI only released details of its reappearance this week after an investigation. The agencies did not return messages seeking comment Friday.

Roosevelt, who was president from 1901 to 1909, apparently had the watch with him at the Battle of San Juan Hill in Cuba during the Spanish-American War and during future exploits, including wild game hunting in Africa and exploring the Amazon in South America, according to the Park Service.

The watch, made by the now-defunct Waltham Watch Co. in Massachusetts, appears like many pocket watches of its day, with a plain silver exterior and no etchings. But the inside reveals its significance, with engraving that says “THEODORE ROOSEVELT” and “FROM D.R. & C.R.R.,” referring to Roosevelt’s brother-in-law and sister, Douglas Robinson Jr. and Corinne Roosevelt Robinson.

When it showed up last year at Blackwell Auctions in Clearwater, Florida, owner Edwin Bailey was excited by the engraving but skeptical it was real. It had no supporting documents with it, and the general mindset among art dealers and collectors is to verify before getting your hopes up, he said.

Bailey said he did not know the watch was stolen, and the person who brought it to him didn’t know where it came from. He declined to identify the person, saying he never divulges the identities of his consigners. He only would say the person was an art dealer and collector in Buffalo in the 1970s and 1980s.

The collector told Bailey that he received the watch from another man who used to borrow money from him to go “picking” for antiques and other collectibles in the late 1980s. The picker would leave the watch with the collector as collateral, Bailey said.

One day, the picker never showed up to retrieve the watch, and the collector found out that he had died, Bailey said.

“This dealer probably had that thing just squirreled away for 30 years thinking it was just another pocket watch,” Bailey said Friday. “I don’t think that my consigner had a clue about not only where it came from, but he probably didn’t even suspect it was real.”

Bailey said he researched the watch for weeks, pouring through Roosevelt’s writings in online archives, trying to come up with definitive proof it was authentic. He said he found several bits of evidence that made him believe it was. The FBI, Park Service and Sagamore Hill officials would later confirm it was the real deal.

In a note to his sister in May 1898, Roosevelt wrote, “Darling Corinne, You could not have given me a more useful present than the watch; it was exactly what I wished. ... Thank old Douglas for the watch — and for his many, many kindnesses.”

He also mentioned a watch in his 1914 book “Through the Brazilian Wilderness.” Writing about a bayou crossing, he said “One result of the swim, by the way, was that my watch, a veteran of Cuba and Africa, came to an indignant halt.” Bailey believes that was the same watch Roosevelt’s sister and brother-in-law gave him.

Bailey also wrote letters and sent pictures of the watch to various museums, the Sagamore Hill historic site and others, asking if they had any information about it.

Last year, shortly before he was set to put the watch up for auction, Bailey got a visit from several people he thought were interested buyers. Then they pulled out their badges and a warrant. It was the FBI coming to investigate the watch and take possession of it, he said.

The federal agents were courteous in asking questions, and he told them the story. He said he was glad that the watch is now where it belongs.

“It was exciting,” Bailey said. “I’ve had a small handful of items that I say ‘these are the best things I’ve ever handled.’ I got to hold something that was personally treasured by a prominent American president. … This was Teddy Roosevelt’s watch. This was a Mount Rushmore guy’s personal pocket watch.”