A criminal investigator is an officer of the law who investigates crimes, interviews witnesses, and helps build cases that lead to an indictment. To start a career as a criminal investigator, you need to have several qualifications, which depend largely on the organization for which you plan to work. At the federal level, organizations like the FBI require you to gain acceptance into and complete an intense, five-month training program. For this level, you need significant law enforcement experience, and most applicants hold a degree in criminal justice or a related field. If you want to become a state or local police officer, you still need to attend a police academy, but the qualifications for admission are lower. You need a high school diploma or a GED certificate. However, it can take some time to work up to the level of a criminal investigator or detective.