In an aggressive bid to recover priceless treasures stolen from the Isabella Stewart Gardner Museum decades ago, the museum’s board of trustees on Tuesday doubled its reward to $10 million and set a hard deadline: Anyone hoping to collect the money must come forward by the end of the year.

“We want our paintings back now,’’ said Anthony Amore, the museum’s security director, who urged anyone with information about the 13 masterworks to contact the museum directly with a guarantee of “complete confidentiality.’’

The announcement reflects growing frustration that the whereabouts of the paintings remain a mystery, 27 years after they were stolen. Despite an exhaustive investigation, none of the pieces has been recovered, and no arrests have been made.

A group of trustees had discussed the possibility of increasing the reward over the past year, museum officials said. The full 23-member board of trustees approved the $5 million increase Tuesday at its annual meeting.

Museum officials said they hope the $10 million reward, which expires at midnight on Dec. 31, will send an urgent message to anyone withholding information about the artwork’s whereabouts, and dispel any doubts about whether they intend to pay it.

“These works of art were purchased by Isabella Stewart Gardner for the ‘education and enjoyment of the public forever,’ ’’ said Steve Kidder, president of the museum’s board. “It is our fervent hope that by increasing the reward, our resolve is clear that we want the safe return of the works to their rightful place and back in public view.’’

Museum officials said most art thefts are either solved shortly afterward, or a generation later, and they hope the heightened reward, and the publicity surrounding it, will help bring the paintings back. The reward, financed by the museum’s trustees, will be paid to anyone — except the thieves themselves — who provides information leading to the return of all 13 stolen pieces, in good condition.

If someone helps recover a portion of the stolen artwork, they can collect part of the reward. And if the artworks remain missing on Jan. 1, the reward will revert back to $5 million.

“To my knowledge, the $5 million reward is the nation’s largest private reward,’’ Amore said. “Doubling it to $10 million is a testament to our commitment to bringing these pieces home to their rightful place.’’

In the early morning hours of March 18, 1990, two men disguised as police officers showed up at the museum’s side door on Palace Road in Boston’s Fenway neighborhood. They claimed to be investigating a disturbance and were let inside by a 23-year-old security guard. The thieves tied up the two guards on duty, and spent 81 minutes cherry-picking an assortment of treasures, from masterpieces by Vermeer and Rembrandt to a bronze eagle finial atop a Napoleonic flag.

Among the stolen items, worth an estimated $500 million, are Vermeer’s “The Concert’’; three Rembrandts, including his only seascape, “The Storm on the Sea of Galilee’’; and works by Flinck, Manet, and Degas.

The heist remains the world’s largest art theft and one of Boston’s enduring mysteries. It has gone unsolveddespite promises of immunity and a lucrative reward, which was increased from $1 million to $5 million in 1997.

Two years ago, the museum announced a new $100,000 reward for the return of the finial, one of the least valuable stolen items. The hope was that someone might have seen the item without realizing its significance, and its recovery could point investigators toward the others.

The FBI and the US attorney’s office have said that efforts to recover the artwork remain a top priority and that the investigation is active. While the five-year statute of limitations on theft charges has long expired, authorities could file charges for hiding or transporting the stolen artwork or lying about it to federal investigators.

“We’re not at a dead end and leads continue to come in all the time, but we also try to think creatively of anything we can do to advance the investigation and this is a welcome development,’’ Acting US Attorney William Weinreb said of the reward. “We hope it works.’’

Kristen Setera, a spokeswoman for the FBI’s Boston office, said in a statement that “works of art hold a special place in our society and the FBI supports the museum’s decision to increase its reward.’’

She urged anyone with information about the location of the paintings to contact the FBI, the museum, “or a third party if you wish to remain anonymous, as soon as possible.’’

Four years ago, the FBI announced it was confident it had identified the thieves – two local criminals who have since died – and determined that the stolen artwork traveled through organized crime circles from Boston to Connecticut to Philadelphia, where the trail went cold around 2003.

In recent years, Robert Gentile, an 80-year-old Connecticut mobster, has been at the center of the FBI’s efforts to recover the artwork. Last year, a federal prosecutor alleged that Gentile offered to sell some of the paintings to an undercover FBI agent in 2015, but the deal collapsed and Gentile was arrested on unrelated gun charges.

Gentile has vehemently denied any knowledge about the whereabouts of the stolen artwork, although he faces the prospect of more prison time when he’s sentenced for the gun charges in August and could go free if he cooperated in the case, according to his lawyer.

“I am heartened to see that the reward has doubled and someone other than my client will be the beneficiary of such generosity,’’ Gentile’s lawyer, A. Ryan McGuigan, said Tuesday.

Investigators in recent years have also renewed their focus on Richard Abath, the guard who buzzed the thieves in.

Two years ago, federal authorities released surveillance video that showed Abath opening the museum’s side door and letting a man in at 12:49 a.m. on March 17, 1990 — 24 hours before the theft — and appealed to the public for help identifying the man.

At the time, law enforcement officials said it appeared suspicious and may have been meant as a dry run.

Last week, in response to inquiries from the Globe, Setera said investigators have identified the man but declined to name him or say whether his admission to the museum is considered suspicious.

“The public’s assistance is no longer needed,’’ Setera said.

Amore, who has worked with the FBI and the US attorney’s office on the investigation since he became the museum’s security chief 12 years ago, said investigators continue to chase leads and hope the increased reward motivates someone to step forward with critical facts, not theories.

“This isn’t a sign that we’ve reached a dead end,’’ Amore said of the reward. “We just hope it’s the thing that prods a person to give us the information that gets us over the hump.’’

Shelley Murphy can be reached at shelley.murphy@globe.com. Follow her on Twitter @shelleymurph.