All it takes is a tiny watermark or pencil scribble to send Sarah Kianovsky pouring over hundreds of files, calling art dealers or catching a plane to Europe.

Kianovsky, an assistant curator at the Fogg Art Museum, is heading up the effort by Harvard University Art Museums to conduct provenance research—tracking the whereabouts of a work of art over time—on pieces in their collections that may have been looted by Nazis during World War II.

The Nazi party conducted the largest art theft in history, Kianovsky says. Although many pieces were returned after the war, The New York Times estimated last week that about 100,000 works are still missing.

Harvard researchers are tracing the ownership of all museum artwork that may have been in Europe between 1933 and 1945. The artwork in question includes more than 500 paintings, 3,100 drawings and 120 modern art objects in the Fogg Art Museum and 750 objects in the Busch-Reisinger Museum.

Kianovsky says art from certain locations and collectors raises red flags.

“Certain dealers who were known to deal in looted art, problematic museum collections and paintings sold in Austria and Germany after 1948 are our main clues,” she says. “It has to do with knowing the progress of the war and the people involved.”

To date, nobody has claimed ownership of any work in Harvard’s museum collections, but Kianovsky stresses the importance of “proactive” research.

“Rather than wait for claims, we decided to take the broader view and look at the history of our objects,” she says.

Elementary, My Dear Watson

Every painting in the museums’ collections has a file recording all of its known owners, every collection in which the piece has been featured, scholarly commentary and a copy of every article and book in which the piece has appeared. Thorough provenance research requires examination of sales records and auctions, exhibition history and the artwork itself, according to Kianovsky.

“It’s really, really slow,” she says. “So many bits and pieces to put together.”

Kianovsky says she began to suspect the authenticity of one painting after noticing a label that resembled the markings of the Kohner collection, a Budapest dealer who sold illegally obtained artwork. Many of Kianovsky’s colleagues at other museums had noticed similar labels in their collections.

After examining the historical whereabouts of the painting, Kianovsky says she thought it might have been stolen from French collector Alphonse Kahn, who lost over 2,000 pieces during the Nazi occupation.

Kianovsky says she wrote to the Kahn family, who gave her access to the family’s archives in Paris. But she found no indication in their files that the piece had been stolen.

Finally, after contacting an American colleague, Kianovsky found a 1947 receipt recording the painting’s shipment to the United States—proof that the painting came back into the hands of the Kahn family after the war.