JTA — The Jewish Museum Vienna reportedly is holding hundreds of pieces that were looted by the Nazis.

Some 500 works of art and 900 books have uncertain provenance, the Austrian daily newspaper Der Standard reported over the weekend.

The pieces with questionable origin were discovered by a screening program that started operating in 2007, according to the newspaper.

In December 2011, the museum hired a part-time researcher to check the provenances of its artworks, Reuters reported.

“Anything that was acquired illegally ought to be returned,” Danielle Spera, who became museum director in 2010, told Der Standard. “There will not be a hint of hesitation.”