Many police officers engage in investigation work as a second job, and may have some insights for you. If you are looking for a specific investigation role in fraud investigation you can contact:
1. FBI
2. US Secret Service
3. US Marshal
4. Law offices that specialize in fraud
5. Private Investigators (many are retired police detectives)
6. Property Insurance companies (Especially those that insure special items)
7. US Dept of Justice
8. Security firms. These often provide private security, but also have licensed investigation services.
All of these have investigators or hire private consultants or investigators ... some require a security background check and/or becoming a police officer, especially if you carry a concealed weapon as part of your job.
About a year ago I hired a Private Investigator for one of our plaza's where a major anchor tenant abandoned the premises with unpaid rent ... he is a retired police officer and if you like I can put you in contact with him. He did a good job for what we hired him to do.