The oil on wood painting by German artist Lucas Cranach the Elder dates back to about 1525. It is worth millions of dollars and once was part of Hitler's private collection, The Washington Post reported.

"We've never had anything like this before," museum representative Thomas Almeroth-Williams told the newspaper. "It's incredibly rare."

The museum said it is looking into whether the painting it bought from a New York art dealer in 1963 might have been looted from Jewish owners by the Nazis, since ownership of the work between 1909 and 1945 has not yet been verified.

A researcher studying Hitler's art collection recognized the painting in a photograph of Hitler's private collection showcased in an album at the Library of Congress in Washington, the Post said. She brought it to the attention of the National Gallery, whose experts determined the painting in the photograph and the work in the gallery's possession are one and the same, Almeroth-Williams said.