Michelangelo Merisi, better known as Caravaggio (1571-1610), is the most famous Italian painter of the baroque period, and the art trade is always looking for lost works by this fiercely original and timeless artist. But new attributions provoke fierce debates.

On Thursday, the art dealer Eric Turquin unveiled a spectacularly well-preserved 17th-century canvas of “Judith and Holofernes” that Marc Labarbe, an auctioneer based in Toulouse, France, found in the attic of a house there in 2014. Mr. Turquin has spent the past five years researching this unsigned painting. He is convinced it is a long-lost masterpiece by Caravaggio.