How can I find information on a missing person from the secret service?

My great uncle was in the secret service working on the atomic bomb project when he went missing. I need to know how I can find information about him. Who would have documents related to his recruitment, projects he may have been involved with, or his release papers. Thank you so much.How can I find information on a missing person from the secret service?write to your congressman, you should get some responseHow can I find information on a missing person from the secret service?Good luck. I don't think you will ever find out this information.