New video could be key to solving $500 million art heist

It appears as though investigators in Boston are closer than ever to solving a 25-year-old mystery behind a $500 million art heist, one of the largest in history.

The release of some never-before-seen video could provide new leads in the case.

From the archives: 1990 Isabella Gardner Museum art heist

It's grainy footage that could help crack the case. The FBI wants to identify the man who entered Boston's Isabella Stewart Gardner Museum the night before the grand heist, raising questions whether it was a dry run.

Thirteen masterpieces were stolen, including Rembrandt's "The Storm on the Sea of Galilee" and Vermeer's "The Concert."

Law enforcement just revealed the two men who robbed the museum are now dead. But they're not releasing their identities.

On March 18, 1990, the men posed as Boston police officers. Security guard Richard Abath buzzed them in a side door and was later found tied up. The robbers pulled out the security video and vanished with the art.

Former Boston Globe reporter Stephen Kurkjian has written a book on the case he covered for two decades.

"The night watchman has somewhere in his cranium the knowledge of the identity of one of the two thieves," Kurkjian said.

For years, the security guard insisted he had never let anyone into the museum after hours. He even passed two lie detector tests.

But as authorities renewed their search for the missing art, old tapes uncovered a different picture. The video shows Abath appearing to buzz a man inside on the eve of the robbery.

"He says he doesn't remember it, but I have to imagine that it's not just a passing visit of that person," Kurkjian said. "It has to be that that person had a relationship with him."

The FBI has long suspected it was an inside job. Special Agent Geoff Kelly spoke to CBS' "Sunday Morning."

"It would be very unusual that they would have rung that buzzer not knowing that they were gonna be let inside," Kelly told "Sunday Morning."

CBS News tried repeatedly to reach Abath, but our messages have gone unanswered.

Meantime, authorities are offering $5 million for information leading to the missing artwork in good condition. Since the statute of limitations has run out, anyone can come forward without fear of arrest.