Originally Posted by
John Venus
Pre Crisis Bruce was the first Robin and became an apprentice to a detective name Harvey Harris who trained him. This was all retconned Post Crisis wherein we shifted from Bruce wanting to be a crime fighter almost immediately after his parents death to him initially just wanting to avenge his parents deaths and later realizing that he needs to have a wider focus.
I think you might be referring to his Pre Crisis origin. He didn't join the FBI but he was planning to while he was in university. However during one of his lectures Bruce got into a debate with his professor about the difference between justice and the law which lead him to abandon the idea entirely. Bruce wanted justice not follow the law blindly.