Depending on whether you want to start your own private investigation agency business or work under the license of another company education may be needed.
If you are working for a company many times there are no education requirements. The company you become employed with typically takes on the responsibility of training their investigators to some extent or another. However many states like Oregon require on-going education to maintain a license in their state.
If you are looking to start your own private investigation agency business, most states require a specific amount of investigative hours in experience, which can be supplemented with college credits. If you have some experience as a private investigator, loss prevention agent, police officer or the equivalent, this experience may be counted as credits towards the needed experience required by most states.
Always check with the state licensing department to be sure of the requirements of that state.