Twenty years ago, two men masquerading as Boston police officers overpowered guards at the Isabella Stewart Gardner Museum on the Fenway and stole 13 masterpieces worth an estimated $300 million. The artwork is still missing and the identity of the thieves unknown. But the FBI is resubmitting some evidence from the crime scene to its lab in Virginia in the hope that advances in DNA technology will reveal new clues.

Twenty years ago today, two men masquerading as Boston police officers overpowered guards at the Isabella Stewart Gardner Museum on the Fenway and stole 13 masterpieces worth an estimated $300 million. The bold theft of works by Rembrandt, Vermeer, Degas, Manet and others has spawned dozens of books and even more theories about who pulled it off and where the artwork is today.

Numerous investigators have tried to crack the case, and several criminals have claimed they could broker a deal for the art’s return in hopes of collecting the museum’s $5 million reward.

The latest twist in the story came earlier this month when the FBI announced that evidence from the scene will be resubmitted to its laboratory in Virginia in hopes that advances in DNA technology will reveal a clue.

The FBI did not reveal what the evidence is, but some experts familiar with the case say it will probably include duct tape used to bind the museum guards.

“There could be skin oil on the tape, and I think that’s what they’re reaching for,” said Charles G. Moore, a Plymouth private investigator who has long pursued the Gardner case.

Even if DNA is found, the problem becomes finding a match for it, he said. Back in 1990, there was no data bank of criminal DNA.

Also this week, the FBI launched a digital billboard campaign along Interstate 93 in Stoneham and Interstate 495 in Lawrence to solicit the public’s help in finding the stolen artworks. The agency is asking anyone with information to contact the FBI’s Boston office and go to www.gardnermuseum.org for more information.

Moore, the investigator from Plymouth, has been successful tracking down other stolen art. Even though the Gardner theft happened 20 years ago, he still devotes some time to the case each week, he said.

He said he has recovered more than $21 million worth of stolen art for collectors, institutions and insurance companies since 1976.

Moore believes the paintings were stolen because they were an easy mark in a poorly guarded building.

“We’ve run down an incredible amount of leads, and I’ve eliminated everyone from my “A” list of suspects,” Moore said.

Even lesser-known works were catalogued and well-publicized, so getting rid of the pieces stolen from the Gardner Museum would have always been difficult, he said.

There’s a chance that, unable to find buyers, the art was destroyed by the thieves. It’s also possible that the thieves died after hiding the paintings, Moore said.

“There’s always a chance that the art will never be found, but it’s improbable,” he said. “There were too many paintings stolen. Something has got to pop up.”

There’s a chance that someone who has no interest in finding them will discover the paintings by accident.

An author has suggested that the art may even be stashed on the South Shore. In his book “The Gardner Heist,” published two years ago, Ulrich Boser puts the art in a retired crook’s beach house in Marshfield or Plymouth, a storage shed in Brockton or a farmhouse in western Massachusetts.

Boser also explores another South Shore connection: While suspects have included everyone from bank robber Carmello Merlino of Quincy, who died in prison in 2005, to Boston thugs David Houghton and Bobby Donati, who are also dead, he suggests that Braintree native David Turner, a convicted robber and murder suspect, was one of the robbers.

Some art dealers and people in law enforcement have wondered whether well-known art thief and Milton native Myles J. Connor Jr. has said all he knows about the Gardner job. He has admitted that he cased the Gardner numerous times in the 1970s.

Connor was not a suspect, since he was in prison at the time of the Gardner heist, but his record of art thefts dates to the 1960s and includes the daylight theft of a Rembrandt from the Museum of Fine Arts in 1975.

Connor and William P. Youngworth III of Randolph, a small-time con artist and felon, once claimed they could arrange for the return of the paintings in return for the $5 million reward and Connor’s release from prison.

Donald Stern, former U.S. attorney in Boston, was interested enough to have Connor transferred from Pennsylvania to a Rhode Island prison so that he could be interviewed. But nothing came of it. Connor was sent back to Pennsylvania and the mystery remains unsolved.

Robert Sears may be reached at bsears@ledger.com.

WHAT WAS STOLEN?

Paintings

Vermeer’s “The Concert,” 1658

Rembrandt’s “The Storm on the Sea of Galilee,” 1633

Rembrandt’s “A Lady and Gentleman in Black,” 1633

Manet’s “Chez Tortoni,” 1878

Flinck’s “Landscape With An Obelisk,” 1638

Objects

Chinese bronze beaker, 13th century B.C.

Napoleonic flagpole finial, 1813

Drawings, sketches

Degas’ program for an artistic soiree (2), 1884

Degas’ “Three Mounted Jockeys”

Degas’ “Cortege aux Environs de Florence”

Degas’ “La Sortie de Pesage”

Rembrandt’s “Self-Portrait,” c. 1634

WHO'S WHO

JAMES “WHITEY” BULGER, 80, South Boston. Boss of Winter Hill Gang. Wanted for 19 murders, drug trafficking, racketeering, extortion. On the run since 1995, when he was tipped off to federal racketeering indictment. Fled with girlfriend Catherine Grieg of Squantum.

Suspected of playing a part in moving stolen Gardner artwork out of United States.

Often stayed at Quincy condo next door to David Turner’s and at Grieg’s Squantum house. Once a secret FBI informant. Whereabouts unknown.

WILLIAM YOUNGWORTH III, 52, Former Randolph antiques dealer. Small-time con artist. Onetime associate of Myles Connor. 

Long rap sheet, including 1997 conviction for possession of stolen van. Said then he could help recover Gardner paintings; investigators came to doubt it. Withdrew offer.

Kept Connor’s art collection at antique shop while Connor was in prison. Connor later accused him of selling collection to pay for drug habit.

On probation since 2004. Convicted in Hampshire County for stealing Palm Pilot, possessing brass knuckles.

DAVID TURNER, 42, Braintree High athlete and 1985 graduate. Thief, legman and enforcer for mobster Carmello Merlino. Suspected of three murders, including killing of best friend Charles Pappas in 1995. Pappas was picked up with Merlino in 1992 and questioned about Gardner theft.

Convicted in 1999 for attempted bank vault robbery. Serving 38 years in federal prison.

Denied any connection to museum heist. Bought condo at Louisburg Square South in Quincy in 1989, next door to Bulger. His lawyer, Martin Leppo of Brockton, sold him the condo.

MYLES CONNOR JR., 66, Milton native. Former rock musician. Convicted art thief, bank robber, drug dealer. Brokered return of a Rembrandt he stole from Museum of Fine Arts in exchange for reduced sentence on unrelated charges.

In federal prison in California at time of Gardner theft. Admitted to casing the museum in 1970s with suspect Bobby Donati.

Claims Donati and David Houghton, both of whom are now dead, told him they did the heist. Frequently claimed he could make a deal to return the paintings. Subject of a book and possible movie.

CARMELLO MERLINO, 70, Died in prison in 2005. Formerly of Quincy and South Boston. Illegal gun dealer, cocaine dealer. Ran auto body shop as front for stolen goods. David Turner’s boss and mentor.

FBI talked to him about Gardner theft several times. He denied involvement but said he could help recover the paintings. Never produced a contact.

Arrested in 1999 with Turner and others for botched bank vault robbery in Easton. Sentenced to 47 years in federal prison in 2002.