Thomas Gavin’s more than two-decades-long crime spree is remarkable by any standard.

He spent the ‘60s and ‘70s pilfering historic artifacts — mostly antique weaponry — from nearly a dozen museums up and down the East Coast. The firearms he filched sat unnoticed and largely unmissed for half a century in a cluttered barn on his Pottstown estate.

And it was only after FBI agents linked him last year to one of those crimes — the 1971 theft of a historic Revolutionary-era flintlock rifle from the Valley Forge National Historical Park — that he volunteered that he’d stolen quite a bit more.

But as it came time this week for Gavin, 78, to own up to his past in federal court, the Montgomery County collector walked away with a sentence almost as remarkable as the stunning breadth of criminality to which he’d confessed: one day in jail.

U.S. District Judge Mark A. Kearney took pains as he imposed that punishment in a four-hour court proceeding Tuesday to note that the sentence also included a one-year term of house arrest, two more years of probation, a $23,485 restitution order, and a $25,000 fine.

Still, the judge marveled at the fact that with Gavin sitting in front of him, having admitted to being one of the most prolific museum thieves in the nation’s history, the sentencing options were limited.

“You were on quite a tear as a young man stealing artifacts and not getting caught,” Kearney said. “But here’s the interesting part: But for your disposal of those items in 2018, you wouldn’t be sitting here. I think that’s a fascinating gap in our criminal justice system.”

The reasons, Kearney noted, were myriad. Statutes of limitations on many of the thefts had expired long ago. In other cases, the items Gavin stole didn’t surpass a $5,000 threshold for the federal crimes with which he might have been charged.

In the end, prosecutors could not charge him with any of the thefts. Instead, Gavin pleaded guilty earlier this year to one count of disposal of an object of cultural heritage stolen from a museum, an offense tied to his attempt in 2018 to sell the rifle he stole from Valley Forge — one of the few surviving works of Pennsylvania master gunmaker John Christian Oerter.

And while that crime carries a maximum penalty of up to 10 years in prison, Kearney cited both Gavin’s advanced age and rapidly declining health as further considerations in his sentence.

But a larger question hung over the proceedings and ultimately went unanswered: Why had the man set out on his historic crime spree in the first place?

Gavin, who spent much of the hearing slumped in a wheelchair next to his lawyer breathing heavily and repeatedly asking his lawyer to repeat what the judge had just said, offered few answers.

“I’m sorry for all this trouble,” he told Kearney. “I never really thought about it back then, and now it’s all come out. I didn’t think it would make a hell of a lot of difference.”

He didn’t do it for money. He held on to his purloined loot for decades, telling no one — not even his family — until he sold much of his collection in 2018 for a pittance. He let the Oerter rifle, valued at $175,000, go for only $4,000.

“Tom is a collector of all manner of old things,” his lawyer Harvey A. Sernovitz said in court filings. “Every square inch of his barn is jammed full with a lifetime of things he bought at barn sales and flea markets … from old typewriters, sewing machines, clocks, steam engines and scales to old cars. … Whether he is considered a collector or a hoarder, profit was not his motivation.”

And there was little in his background to explain his thievery.

He enrolled at Drexel University at 16 to pursue an electrical engineering degree, but dropped out to build a successful career as a renovator of historic properties and estates.

He married his high school sweetheart, raised three children, and earned national accolades for his work on projects including a $17 million restoration of a historic plantation in Virginia.

And yet, still, Gavin spent his 20s and 30s striking at museums again and again.

Prosecutors said Gavin’s haul included dozens of historic rifles, pistols, revolvers, and other artifacts from the 18th and 19th centuries — including a rifle once owned by naturalist John James Audubon taken from the Academy of Natural Sciences in Center City.

Most of the museums he targeted — including the Philadelphia Museum of Art, the American Swedish History Museum, the Hershey and Mercer Museums, and institutions in New Jersey, Virginia, and New York — either hadn’t noticed their items were missing or had no records today of those thefts. Gavin has since helped identify many of the weapons, enabling the FBI to return them, Assistant U.S. Attorney K.T. Newton said.

But few of his scores were as notable as the Oerter rifle.

Historians have described the firearm — a five-foot rifle, stamped with its 1775 date of manufacture, decorated with a brass-wire fleur-de-lis on the wrist, and engraved on its stock with the name “W. Goodwin” — as a prime specimen of a late-18th-century style known as the “flintlock Kentucky long rifle,” popularized by frontiersmen like Daniel Boone.

The guns proved instrumental in the Revolutionary War effort, allowing colonial soldiers to shoot more accurately and from farther away than their British counterparts, who carried smooth-bore muskets.

Oerter ran one of the busiest gun manufacturing outfits in Christian’s Spring, near present-day Nazareth, during the war. Known for their elaborate silver and brass wire inlays and carved decorations, his weapons are recognized by arms scholars as some of the finest and most important of the period.

The one Gavin stole from Valley Forge is one of only two signed and dated examples of Oerter’s work known to still exist.

It was a Boy Scout touring the park with his troop that noticed the rifle had vanished from its aquarium-size glass display case in 1971.

At the time, investigators determined the thief had entered the visitor center just after 9 a.m. on the morning of Oct. 2, forced open the case with a crowbar, and walked out with the weapon while a receptionist sat unaware in the next room.

Authorities waited nearly a month to publicly disclose the heist out of fear that whoever had taken the gun might panic when its value was realized and destroy it.

But the trail quickly ran cold.

In court Tuesday, Gavin explained that his trouble began nearly five decades later when he attempted to give the rifle away in 2018 as a gift to a doctor in Gladwyne, whom he warned of its problematic provenance.

“I said, ‘Look there could be a problem with this thing,’” Gavin recalled. “He said, ‘Don’t worry about it. We’ll sell it overseas.’”

The next thing he knew, Gavin said, an antiques dealer, Kelly Kinzle, of New Oxford, showed up at his door. Gavin told the judge that he assumed the doctor had sent Kinzle and accepted the dealer’s offer of $4,000 for the Oerter rifle and sold him several other items from his collection.

It was Kinzle who first identified the distinctive firearm as stolen. He hired lawyers to help him investigate the theft, alert the FBI, and eventually negotiate its return.

Kearney thanked Kinzle in court Tuesday for his efforts — many undertaken at his own expense — to investigate the theft and enable the rifle’s repatriation.

Gavin, meanwhile, was wheeled off after the hearing concluded to serve the rest of the day in jail.