John Demjanjuk, the former Ohio auto worker charged in Germany last week, is the defendant in the so-called last great Nazi war-crimes trial but Nazi hunters contend there are many more Nazis living quiet suburban lives, Zuroff told the New York Post.
"We don't have much longer," Zuroff said, "We have to go after them or they will be too sick to bring to trial."
Like Demjanjuk, many of the Nazis are elderly men who worked and raised families in the United States.
Eleven suspected Nazis are currently being prosecuted, the newspaper said. More than 100 have been prosecuted since 1979, with around 60 being deported, and 30 are under investigation.
Investigations can take years. Eli Rosenbaum, the director of the U.S. Department of Justice office that handles Nazi prosecutions, called them the ultimate cold cases.