BOSTON (CBS) – Boston Mayor Marty Walsh said an investigation is ongoing after Boston Public Library officials revealed that two pieces of valuable art are missing.

Boston Public Library President Amy Ryan said an engraving by Albrecht Dürer and an etching by Rembrandt are both missing, though she hopes they have been misplaced.

READ MORE: 3 People Rescued From Rip Currents At Salisbury Beach; DCR Issues Warning

Boston Police Commissioner William Evans said investigators are open to the possibility that the art may have been stolen as a part of an “inside job.”

“We turned it over to the anti-corruption unit on the possibility it’s an inside job with an employee of the library. That’s it at this point I can’t go into a whole lot of detail,” Commissioner Evans said.

“We’re looking at some, can’t go into detail, but looking at a few people inside who might have had access to that particular area,” Evans said.

Walsh said he wishes information about the missing paintings had come out earlier.

“It’s been misplaced for anywhere from six months to a year. You don’t misplace $600,000 pieces of art. I’m not happy about it and wish it came to light sooner,” Walsh said.

The Dürer etching is called Adam and Eve, and a similar print recently sold at auction for more than $600,000.

A Rembrandt self-portrait with an estimated value of $20,000-$30,000 is also missing.

Boston police said they were called on April 29 about the paintings, which had been determined to be missing by library officials on April 8.

Walsh said that regardless of whether the art was stolen or misplaced, he wants to increase security at the library.

WBZ-TV’s Jim Armstrong reports

READ MORE: Worcester Unveils Statue Of Celtics Legend Bob Cousy

“I know we do have cameras in the library,” Walsh said. “It’s an ongoing investigation on one hand. On the other hand we have to tighten up security to make sure we protect the assets we have.”

A Rembrandt self-portrait. (Photo credit: ATTILA KISBENEDEK/AFP/Getty Images)

A Rembrandt self-portrait. (Photo credit: ATTILA KISBENEDEK/AFP/Getty Images)

Newbury Street’s Galerie d’Orsay currently has three Rembrandt prints for sale, and has sold others by Dürer in the past. Galerie d’Orsay’s owner, Sallie Hirshberg, told WBZ-TV’s Jim Armstrong the prints missing from the the library are what are known as “lifetime works,” meaning Rembrandt and Dürer actually created these prints themselves.

“Hopefully they have some sort of collector’s stamp on them so they can be identified and they won’t be able to be resold. But if they don’t then there’s a possibilty that they could be available on the market again.”

“I hope, like everyone else, that they are found and that they’ve just been misplaced because they’re just little treasures.”

Hirshberg said that when she heard of the missing pieces, she thought to herself, “another terrible loss for Boston after the Isabella Stewart Gardner museum.”

That’s where three Rembrandts were stolen back in 1990 – and remain missing.

“It’s an amazingly difficult challenge to secure a public institution, especially when the goal of that institution, whether it be a library or a museum, is to give the public great access to the world’s greatest works,” explained the Anthony Amore, the Gardner’s Security Director.

“Whenever you say our goal is to take these rare or important pieces and let the public have access to them, then you’re creating a greater security challenge.”

Amore says he’s confident that Boston Public Library kept good records of everyone who had access to the prints, but he’s also hopeful they’re just lost.

“If they believe there’s a possibility they’re still in the building, I have no reason to doubt what they say,” Amore said. “However, art theft is an enormous worldwide problem, numbers in the billions.”

MORE NEWS: Boston Marks Juneteenth With Celebration In Nubian Square

WBZ NewsRadio 1030’s Carl Stevens reports