He is best known for his anti-Nazi spying activities in the 1930s and 1940s which resulted in the successful prosecution of American Nazis and the prevention of dozens of acts of sabotage and assassinations.
At around the same time he and his uncle, Julius Klein, started the anti-Nazi newspaper the National Free Press and began spying on local Nazi groups.
Roos moved to Los Angeles in 1934 to work in Hollywood as a writer and editor, but he found himself again involved in anti-Nazi spying, which included infiltration of the Silver Legion of America, the Ku Klux Klan, the isolationist group America First, and the German American Bund.
[5][4][3] Along with his secret activities before and during the war, Roos also developed a national profile with his publication of the News Research Service newsletter, which published a continual stream of documentation of Nazi activity in the U.S. and allowed him to develop close relationships with politicians, activists, and journalists such as Walter Winchell.
[4][3] In 1950, Roos became the executive director of the Jewish Federation Council of Greater Los Angeles, Community Relations Committee, succeeding Leon L. Lewis, and was involved with social and political issues in Southern California such as school busing, discrimination, and prayer in schools.