The following resources outside the Library of Congress are freely available online and are useful to locate general information about Jewish composers.
The Biographies section of the website contains biographical sketches of many historical and living composers of concert music.
"This website is focused on . . . the role of music in the Holocaust—the systematic mass murder by the Nazis of six million European Jews, as well as homosexuals, communists, Roma, and other victims during the Second World War. The articles describe the wide range of musical activities that took place in camps and ghettos across Nazi-occupied Europe, focusing not only on the work of professional musicians and composers but equally on the music created and performed by millions of ‘ordinary’ people in response to their experiences of internment. The recordings include songs written and sung by victims across Europe, as well as full-length compositions written primarily in Theresienstadt and in the post-war period." (https://holocaustmusic.ort.org/about-us/)
The Bibliography section of the website contains numerous journal articles and book titles of interest to those researching Jewish composers. The People section includes biographical sketches of Jewish composers active before and during the Holocaust in Europe.