A million-dollar painting by 17th century Dutch artist Frans van Mieris was stolen from an Australian state gallery in a quick and expert raid during viewing hours, officials said Thursday. Police suspect an inside job.

Police said they had questioned gallery staff and had not ruled out one or more of them being involved in stealing "A Cavalier (self portrait)" from the Art Gallery of New South Wales state sometime on Sunday.

An FBI art theft expert said that the painting would likely disappear for years if police failed to catch the thieves, and that authorities had a poor record in such cases involving well-known works.

Gallery director Edmund Capon said the tiny painting — 11 inches by 16 inches, including a wooden frame — was unscrewed from a wall in a small display room that was not fitted with security cameras.

"To be honest, I could slip it under your coat ... It could have happened that way," Capon said at a news conference, estimating it would have taken the thief a few minutes to remove the wall screws.

The painting was insured for $1.17 million, he said.

Police were scouring recorded images from cameras in other parts of the gallery for clues to the heist, which they believe occurred between 10 a.m and 12:30 p.m. local time.

They appealed for witnesses to come forward, estimating that up to 6,000 people visited the gallery Sunday.

"The artwork was expertly removed from the art gallery wall, and we're following all lines of inquiry to establish who is responsible for the theft," said Police Superintendent Simon Hardman.

Van Mieris was a Dutch Baroque-era painter whose works are included in The Royal Collection in London and displayed in the J. Paul Getty Museum in Los Angeles, among others.

Rendered in oil paint on an oak panel, "A Cavalier" — a French word meaning a chivalrous gentleman — depicts a man, believed to be the artist, seated on a chair and dressed in a feathered hat and frilled sleeves.

Gallery staff reported the theft to police on Monday, after searching the archives in case it had been misplaced, Capon said.

Security measures, which include foot patrols of the gallery but no screening at the entrance, were being reviewed, he said, adding that he believed it was the first time a piece of art had been stolen from the gallery.

FBI expert Robert Goldman said news of the theft would spread around the world very quickly.

"The FBI experience is that approximately 80 percent of museum theft cases of art are inside jobs," Goldman told Australian Broadcasting Corp. radio from Philadelphia.

No reputable gallery or museum would buy the stolen painting, meaning it would likely be bought by an unscrupulous private collector or remain in the thieves' hands because they would not be able to sell it, he said.

"We find a lot of paintings do eventually come back," Goldman said. "Unfortunately it takes a long time. It can be a generation, but they come back when a spouse or a descendant or an heir attempts to sell the paintings in 25 years or so."

In one famous case, masked gunmen seized the Edvard Munch masterpieces "The Scream" and "Madonna" from a museum in Oslo, Norway, in 2004. The paintings were recovered by police last year, and three men were charged with the heist. Police have refused to say how they recovered the paintings.