Why did the German soldiers come to the Johansens' apartment looking for the Rosens?

In Lois Lowry's book Number the Stars, the Johansens are a family living in Nazi-occupied Copenhagen, Denmark. For a while, they have to pretend that their daughter Annemarie's young friend, Ellen Rosen, is actually their recently deceased daughter Lise in order to stop her from being captured by Nazi soldiers. Eventually, Ellen and her family, along with other Danish Jews, are safely smuggled to Sweden, which was a neutral country in World War II.

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In Lois Lowry's book Number the Stars, the Johansens are a family living in Nazi-occupied Copenhagen, Denmark. For a while, they have to pretend that their daughter Annemarie's young friend, Ellen Rosen, is actually their recently deceased daughter Lise in order to stop her from being captured by Nazi soldiers. Eventually, Ellen and her family, along with other Danish Jews, are safely smuggled to Sweden, which was a neutral country in World War II.

Annemarie and her family have many scary run-ins with Nazi soldiers throughout the book. Probably the scariest is when Nazi soldiers come to the Johansen's apartment in the middle of the night looking for the Rosens— they believe that the Johansens are hiding the entire Rosen family. These soldiers have begun rounding up the Jewish people of Denmark in order to "relocate" them to some unknown place, and the Rosens are on their list. Luckily, Ellen is the only one there, and Mr. Johansen is able to convince the soldiers that Ellen is his daughter, Lise.