Stonington — Last year, Stephen Kurkjian may have been close to a break in the decades-long mystery of the missing masterpieces from the Isabella Stewart Gardner museum in Boston.

While reporting his book, the 2015 investigative thriller "Master Thieves," Kurkjian spent three days interviewing Robert Gentile, the man the FBI believes may know what happened to the paintings.

Almost 25 years earlier, in the early hours after St. Patrick’s Day 1990, two men in police uniforms tied up the night guards at the museum and made off with $500 million worth of art including Rembrandt’s “The Storm on the Sea of Galilee,” and Vermeer’s “The Concert.”

Gentile, just out of prison on a drug charge last year and talking to Kurkjian from his Manchester home, denies he knows where the paintings are, Kurkjian told a packed house at the La Grua Center on Sunday night.

But on a hunch that Gentile may have been a part of an effort to steal the paintings and use them as collateral to get another gangster out of jail, Kurkjian went out on a limb. He made Gentile an offer: come clean and co-write a book about the paintings, then share in the proceeds.

Gentile put his head down for several seconds, and Kurkjian thought he would have a eureka moment in the case that has tormented police and Boston art lovers for nearly 25 years.

His answer was "no," but Kurkjian, who has covered the Gardner heist for more than 20 years in his career as a reporter for the Boston Globe, had enough after three days with Gentile to add to "Master Thieves" and tell most of the story of the missing art.

Kurkjian was a founding member of the Globe’s Spotlight investigative reporting team and is briefly portrayed in the new movie "Spotlight," about the group’s work to uncover widespread sexual abuse by local Catholic priests.

The tale of the Gardner museum heist had captured his imagination for many years, he said Sunday.

“This is a hell of a story,” he said to a crowd that spilled out of the lecture hall’s doors.

Despite a $5 million reward and two decades of work by federal investigators, the paintings’ frames still hang empty on the museum’s walls.

Their loss is a tragedy, Kurkjian said, largely because it violated the vision of Isabella Stewart Gardner, the Boston aristocrat and art collector who traveled all over the world to bring high-quality art back to Boston.

“She wanted to bring that appreciation to us, Bostonians,” Kurkjian said. “She wanted to have America have a great tradition in art.”

But Gardner’s will included a demand that nothing be changed in the museum following her death, so the security system was rudimentary. The two security guards – a member of a rock band who was often drunk or high at work and a music student who used the empty museum to practice trombone – didn’t help things, either.

The thieves smashed the paintings, cut them out of their frames, and put them with several other pieces of art into a getaway car and escaped. The FBI agents investigating the case haven’t had a single glimpse of them since, Kurkjian said.

Whether it was a scheme among Boston gangsters to exchange art for lighter prison sentences, as Kurkjian believes, he said the art won’t be found unless an appeal is made to the Boston public to pass along tips.

Whoever may know the paintings’ fate, he said, is likely a member of the city’s working class, uninterested in the work of the police or museum directors.

Someone like Boston Mayor Marty Walsh or Cardinal Sean P. O’Malley, respected figures among Boston’s “have-nots,” need to make it clear what the city is missing in those empty frames – the gift of public art that Gardner gave the city when she started the museum, he said.

“That loss has to be felt by all of us,” he said. “We have to feel this loss like it was taken from us.”

Kurkjian’s book doesn’t solve the mystery, but he seemed confident Sunday night that the answer would be found.

“The last chapter needs to be written, and that’s the recovery,” he said.

m.shanahan@theday.com

Twitter: @martha_shan