Investigative units can be complex and if your agency is large the units are many and specialized. In large departments, first responding uniformed officers rarely work a case past their initial contact and report. Once a scene is secured and the preliminary information has been gathered, the officer turns the scene and case over to their supervisor or to an investigator. Therefore, it is difficult for a uniformed officer to plan and prepare for a transfer or promotion into an investigative unit. These officers will be interviewing for an investigative position with little experience past a preliminary investigation. However, they have more opportunities to promote, along with more training once they reach the unit.
In contrast, it's not unusual for officers serving in small departments to work a case from first responder and preliminary investigation, to composing and obtaining warrants and subpoenas, and filing charges with the DA's office. Some can work the case through with the guidance of an investigator. These officers have a slight edge in experience, but fewer opportunities to promote. Most training occurs in the field.
Although departments and their units are unique, with a good foundation in basic investigation and ongoing training an officer can obtain the tools and skills that will assist them in the field of investigative work.
There are numerous ways to prepare ahead of time so when the opportunity comes you can at least be familiar with the process, terms, and procedures. In most departments not only do you need to prepare for an oral review board, you should also prepare for a written test. There's an online pre-test you can take to gauge how much you know … or how much you think you know, but don't. Take a pre-test, find your weakness, and obtain training in those areas.
Some examples of low cost, online, law enforcement continuing education and seminars specifically for investigators include case tracking, interview/interrogation, written/verbal analysis, composing warrants and subpoenas, computer/smartphone forensics, digital evidence, arrest/search and seizure/asset forfeiture, and case law/legal issues.