Artworks by Jackson Pollock and Andy Warhol have been stolen from a museum in Scranton, Pennsylvania in the early hours of Friday morning.
Pop artist Andy Warhol died in 1987
The 1949 Pollock oil painting, Springs Winter, was on loan from a private collector and is estimated to be worth about $11.6 million (£6.7m).
Thieves broke a glass door at the back of the Everhart Museum, officials said.
Surveillance cameras were not working at the time of the robbery and police said they had no immediate leads.
A marquee covering the museum's back entrance may have aided the thieves.
The silkscreen by Warhol, entitled Le Grande Passion, has been valued at approximately $15,000 (£8,760).
The pop art icon created the work in 1984 as part of an advertising campaign for a brand of cognac. It was owned by the museum.
Police responded when the museum's alarm system was set off at 0230 local time. Further investigation revealed that the two artworks had been removed from the museum's second floor exhibition hall.
The museum did not comment on why surveillance cameras were not working.