BOSTON (AP) — The two men suspected of masquerading as police officers to rob an art museum of $500 million worth of masterpieces in 1990 are dead, the FBI says.

Two years ago, investigators announced they knew who stole 13 works — including paintings by Rembrandt and Vermeer — from Boston's Isabella Stewart Gardner Museum, but they refused to elaborate, saying only that the investigation was focused on recovering the artwork.

Now Peter Kowenhoven, the FBI's assistant special agent in charge in Boston, has settled the lingering mystery of why no one was ever charged in the biggest art heist in U.S. history: The suspects are dead.

"The focus of the investigation for many years was: Who did this heist? And we have through the great investigative work identified who did this heist, and both those individuals are deceased," Kowenhoven told The Associated Press in an interview. "So now the focus of the investigation is the recovery of the art."

He declined to identify the men.

On Thursday, authorities released grainy surveillance video from the eve of the heist and appealed anew to the public for help in finding the artworks.

The low-resolution video — captured by museum security cameras — shows a security guard appearing to hit an intercom button, then to grant access to a man who can be seen in the museum's reception area at about 12:49 a.m. on March 17, 1990, almost exactly 24 hours before the heist.

The man is also seen getting out of a car matching the general description of one reported to be parked outside the museum minutes before the theft. He uses the same rear entrance as the thieves, according to the office of U.S. Attorney Carmen Ortiz, which released the video.

The stolen artwork includes Vermeer's "The Concert" and Rembrandt's "The Storm on the Sea of Galilee."

A $5 million reward has been offered by the museum for information that leads to the recovery of the stolen artwork in good condition.

With the thieves dead, "all our investigative efforts are being guided toward the recovery of this art," Kowenhoven said.

"Every lead, every tip, everything that comes in we're tracking down, no matter where it is in the area, in the country or overseas," he said. "These are cultural masterpieces that we are trying to recover."

Authorities have said that on March 18, 1990, two men dressed in Boston police uniforms gained entrance to the museum by telling the security guard at the watch desk that they were responding to a report of a disturbance.

Against museum policy, the guard allowed the men into the museum. The thieves handcuffed the museum's two guards on duty and put them in separate areas of the museum's basement.

In 2013, authorities said the robbers belonged to a criminal organization based in New England and the mid-Atlantic states and took the art to Connecticut and the Philadelphia region in the years after the theft. They were offered for sale in Philadelphia about a decade ago and have not been seen since, the FBI said at the time.

___

Online: https://www.youtube.com/watch?v=qSw1BsJEBB4